Health care quality in nonparametric efficiency studies: a review

Scientists are increasingly considering quality in nonparametric frontier efficiency studies in health care. There are many ways to include quality in efficiency analyses. These approaches differ, among other things, in the underlying assumptions about the influence of quality on the attainable efficiency frontier and the distribution of inefficiency scores. The aim is to provide an overview of how scholars have taken quality into account in nonparametric frontier efficiency studies and, at the same time, to address the underlying assumptions on the relationship between efficiency and quality. To this end, we categorized empirical efficiency studies according to the methodological approaches and quality dimensions and collected the quality indicators used. We performed a Web of Science search for studies published in journals covered by the Science Citation Index Expanded, the Social Sciences Citation Index, and the Emerging Sources Citation Index between 1980 and 2020. Of the 126 studies covered in this review, 78 are one-stage studies that incorporate quality directly into the efficiency model and thus assume that quality impacts the attainable efficiency frontier. Forty-four articles are two-stage studies that consider quality in the first and the second stage or the second stage only. Four studies do not assume a priori a specific association between efficiency and quality. Instead, they test for this relationship empirically. Outcome quality is by far the most frequently incorporated quality dimension. While most studies consider structural quality as an environmental variable in the second stage, they include outcome quality predominantly directly in the efficiency model. Process quality is less common.


Introduction
Differences in the cost of providing comparable health care services have led to efforts to identify efficient service providers. Efficiency in service provision can be measured by the ratio of actual to best practice productivity, which, in turn, corresponds to the ratio of output (e.g., case-mix adjusted inpatient days) to input (e.g., health care personnel). Many studies have used nonparametric frontier techniques, such as Data Envelopment Analysis (DEA), to assess the efficiency of the health care sector (see, e.g., Hollingsworth et al. 1999;O'Neill et al. 2008;Kohl et al. 2018). Nonparametric frontier methods can simultaneously consider multiple inputs and outputs without knowing their functional relationship and the factor and product prices. Service providers are rated efficient if they produce a given output with minimum input or the maximum output for a given input, compared with other service providers.
However, the health economics literature increasingly recognizes that in addition to concentrating on quantity input and quantity output, health care quality requires due consideration. According to Donabedian (1980), health care quality consists of structural, process, and outcome quality. Portela et al. (2016, p. 727) outline that the structural quality represents the characteristics of the health care setting, the process quality refers to the various actions undertaken by health care staff in providing care, and the outcome quality reflects the effects of health care services on the patient's condition. While there are other classifications of quality, 1 we used the Donabedian framework to apply a definition of quality that was valid through the overall review period.
Incorporating quality in efficiency studies is a complex task. As early as 1970, Newhouse (1970) elaborated that quantity and quality are goods that compete for resources. A service provider has to compromise between increasing the quantity and increasing the quality given scarce resources. Many studies investigated the theory of the efficiency-quality trade-off in the health care sector. In their review, Hussey et al. (2013) examined the cost-quality trade-off with the following, almost evenly distributed results: around one-third of studies (34%) identify a positive or mixed-positive relationship, slightly less than one-third (30%) reveal a negative or mixed-negative relationship, and somewhat more than one-third (36%) show a lack of, an ambiguous, or a mixed association. The results in the nonparametric efficiency literature are also diverse. While Kooreman (1994) finds evidence of a trade-off between efficiency and quality in assessing the efficiency of Dutch nursing homes, Laine et al. (2005) identify mixed results regarding the association between different quality indicators and the efficiency of (hospital and nursing care) wards providing long-term care in Finland. Pross et al. (2018) reveal a trade-off between quality and efficiency in providing stroke care in German hospitals. Khushalani and Ozcan (2017) do not find evidence of any efficiency-quality trade-off when investigating the efficiency of US hospitals.
The mixed results regarding the relationship between efficiency and quality in nonparametric studies are not surprising, as the studies differ significantly in several factors. First, the units of analysis (decision-making units, DMUs) differ in terms of type (e.g., hospitals, nursing homes) and aggregation level (e.g., facility level, ward level, patient-level). Second, the studies cover different quality dimensions and use varying quality indicators. Third, the DMUs operate in different environments, each of which can have other effects on efficiency. Fourth, the model specifications, most of which assume a priori a specific efficiency-quality association and are tailored to particular research questions, vary. Study results are therefore less likely to be generally valid. Instead, they require interpretation in the respective context. The paper, however, aims to provide an overview of how scholars have integrated quality into efficiency studies. In particular, the aim is to classify nonparametric efficiency studies according to model specifications and quality dimensions. For that purpose, we slightly modified the classifications of Bȃdin et al. (2014) and Varabyova et al. (2017) to particularly pay attention to the model-inherent assumptions about how quality affects efficiency. In addition, the paper gives a comprehensive overview of the quality indicators considered in nonparametric efficiency studies.
The remainder of this paper is as follows. Section 2 describes the data sources, search strategy, and study selection. It also presents the classification scheme and discusses the quality dimensions. Section 3 describes the results. Section 4 discusses challenges when considering structure, process, and outcome quality in efficiency studies. The paper concludes with a summary and a discussion of limitations in Sect. 5.

Data sources, search strategy, and study selection
The selection of the relevant publications was as follows. We performed a Web of Science (WoS) keyword search to identify nonparametric frontier efficiency studies related to health care. We searched for studies published between 1980 and 2020 covered by the Science Citation Index Expanded, the Social Sciences Citation Index, and the Emerging Sources Citation Index. We then examined the studies' keywords and abstracts for nonparametric frontier techniques. We recorded the model specification, the type of DMU, the quality dimensions, and the quality indicators for those studies that applied nonparametric methods. Finally, we analyzed cross-references to studies not covered by the WoS search for possible inclusion in the review.

Classification of model specifications
We used a classification that accounts for a priori assumptions regarding the relationship between efficiency and quality. To that end, we slightly modified the categorization proposed by Bȃdin et al. (2014) and Varabyova et al. (2017). First, we distinguished between Category 1 (CAT1) and Category 2 (CAT2) studies. In contrast to CAT2, CAT1 comprises studies that make a priori assumptions about the association between quality and efficiency. CAT1 consists of one-and two-stage approaches referred to as Class 1 and Class 2 studies.
Class 1 studies incorporate quality directly into the efficiency model, assuming that quality affects the attainable efficiency frontier. In contrast to the conventional classification in the literature, we only classified studies that consider quality in the second stage as Class 2 approaches because the focus is on how quality is incorporated. Put differently, when a paper accounts for other than quality-related factors in the second stage, we classified the article as a Class 1 study. Accounting for quality in the second stage corresponds to assuming 2 that quality affects the inefficiency distribution and, thus, the separability between the input-output-space and the quality-space to hold (Simar and Wilson 2007).
Class 1 includes the five subcategories (SubCATs) 11 to 15 (Table 1): • SubCAT 11 studies incorporate quality in the efficiency model, assuming that efficiency (measured in input and output quantities) and quality are related performance dimensions. • SubCAT 12 studies use separate models for efficiency and quality. This approach understands efficiency and quality as distinct performance dimensions, resulting in an efficiency (also called managerial) model and an effectiveness (also called clinical) model (see, e.g., Portela et al. 2016). • SubCAT 13 covers effectiveness studies that only incorporate quality outputs.
• SubCAT 14 studies examine the effect of quality on efficiency by using separate models. In contrast to SubCAT 12, however, they account for quality and efficiency in one specification while omitting quality in the other. • SubCAT 15 comprises Network DEA (NDEA) studies that overcome the black-box assumption regarding transforming input into outcome (final output). NDEA allows decomposing the overall service production process into two or more corresponding subprocesses. In the health care context, NDEA provides the opportunity to model the input-output-outcome relationship.
Class 2 studies comprise the six SubCATs 20 to 25 (see Table 1 for details). SubCAT 20 has no equivalent in Class 1 since these studies do not take quality into account directly in the efficiency model (i.e., the first stage), but only in the second stage. The model specifications in the first stage in SubCATs 21 to 25 correspond to those in SubCATs 11 to 15. Class 2 papers examine the relationship between efficiency (or effectiveness) and (further) quality indicators in the second stage, mainly using regression analysis. The inclusion of quality in a second stage corresponds to the assumption that quality affects the distribution of inefficiencies; that is, the separability condition holds.
In contrast to CAT1, CAT2 studies do not a priori assume a specific association between efficiency and quality. Instead, these studies rely on conditional efficiency so that "the data themselves tell if and how they (the data or quality in our case; author's note) affect the performance" (Daraio and Simar 2007b, p. 14). Bȃdin et al. (2012) illustrate that regressing the ratio of conditional to unconditional efficiency on quality identifies an influence of quality on the efficiency frontier while regressing conditional efficiency on quality reveals any impact of quality on the inefficiency distribution.

Quality dimensions and corresponding indicators
According to Donabedian (1980), structural quality represents the characteristics of the health care setting, represented by indicators such as appropriate equipment and staff mix. Structural quality is patient-invariant. The possibilities of including structural quality in efficiency studies are pretty heterogeneous. It is possible to incorporate structural quality as a nondiscretionary input (Banker and Morey 1986), desirable output, or a quality-adjusted output directly in the efficiency model.
Clinical guidelines and good clinical practice are often the basis for process quality indicators (Quentin et al. 2019, p. 39), which vary across patient characteristics. Efficiency studies usually incorporate process indicators as outputs. However, medical care consists of several successive phases. Therefore, process indicators can be considered output (referred to as intermediate output or throughput) in one phase and input in the other. Process quality can also be associated with managerial aspects of medical care.
For patients, outcome quality is probably the most crucial quality dimension. Subjective and objective indicators can depict outcome quality. While subjective indicators reflect the opinions and assessments of patients, objective indicators rely on measurable data such as mortality rates, readmission rates, and complication rates. Outcome quality is the final output of providing medical care and also depends on patient characteristics.
Some studies incorporate outcome quality using desirable, others using undesirable indicators. 3 For desirable indicators, such as the patients' willingness to recommend a hospital, the higher the quality, the better. For undesirable indicators such as the mortality rate, there is an inverse relationship between quality and efficiency. Incorporating undesirable indicators into a nonconditional nonparametric efficiency model requires special attention. The literature broadly distinguishes between approaches with and without data transformations (see, for example, Liu et al. (2010) and Halkos and Petrou (2019) for a summary of the range and characteristics of the different approaches).
Models with data transformations include the linear additive transformation suggested by Ali and Seiford (1990), Pastor (1996), andScheel (2001), and the nonlinear multiplicative transformation proposed by Lovell et al. (1987) and Golany and Roll (1989). Models without data transformation involve treating undesirable outputs as desirable inputs (Liu and Sharp 1999), assuming undesirable outputs to be weakly disposable (e.g., that an increase in quality comes at the expense of quantity outputs), and using sophisticated efficiency metrics (e.g., directional distance functions (DDFs), slack-based models (SBMs), and hyperbolic measures).

Results
The WOS keyword search, which covered the period from 1980 until 2020, 4 yielded 504 studies. After analyzing keywords and abstracts, 119 of 185 nonparametric efficiency studies included at least one quality indicator. Reference mining and ad hoc searches returned seven additional articles so that the review finally covers 126 studies. Table 2 provides the complete list of reviewed studies, including their categorization. We included the categorization because, in some cases, multiple classifications would have been possible. The same applies to the assignment of the indicators to the corresponding quality dimensions. In this regard, we refer the interested reader to the Appendix. Table 3 shows the categorization of the studies. 122 studies (97%) belong to CAT1, leaving four CAT2 studies. This finding is not surprising. After all, conditional efficiency approaches are relatively new methods. Within CAT1, 78 studies are one-stage approaches (Class 1). SubCAT 11 (27 papers) dominates in this class. Effectiveness studies (SubCAT 13: 23 papers) build the second most common subcategory, followed by studies that examine the effect of quality on efficiency by adding at least one quality indicator in a separate specification (SubCAT 14: 17 papers). Studies that understand efficiency and quality as distinct dimensions (SubCAT 12: 5 papers), and NDEA studies (SubCAT 15: 6 papers), are less common.

Studies by decision-making units
We built eight categories for DMUs (see Table 3). These include country studies (COUN); regional studies (health regions; HR); hospital studies (hospital care; HC); nursing and aged care studies (nursing care/aged care; NC/AC); studies devoted to the primary health care sector (primary health care; PHC); cross-provider studies covering the hospital and the nursing care sector (HC plus NC/AC) and the hospital and the primary health care sector (HC plus PHC); and a collective category (OTHER), which covers the remaining studies. Hospital studies, including cross-provider studies, make up half of all studies (63 papers). Regional studies that investigate the efficiency of regions, provinces, districts, regional health boards and authorities (20 papers), nursing and aged care studies (16 papers), and primary health care studies (11 papers) together account for some 37% of all studies. Country studies (9 papers) and the residual DMU category "Other" (7 papers), which covers insurances, laboratories, physicians, and patients, make up the smallest DMU groups. Figure 1 shows at least five studies per year since 2011. Before the year 2000, however, we identified only seven studies. These studies include those of Morey et al. (1992), Morey et al. (1995) and Ferrier and Valdmanis (1996), each investigating hospital efficiency; Kooreman (1994) and Kleinsorge and Karney (1992), who assess nursing efficiency; and Thanassoulis et al. (1995) and Salinas-Jiménez and Smith (1996), who evaluate health regions. There are no CAT2 studies before 2013.

Studies over time
Breaking down the evolution of studies by SubCATs (Fig. 2) shows an increase in

Studies by quality dimensions
Following Donabedian (1980), seven possible variants exist for taking quality into account in nonparametric efficiency studies. These include considering single dimensions (S, P, O) and the combinations thereof (SP, SO, PO, SPO). The figures in Table 4 read as follows. Only in Class 1 (78) and Stage 2 of Class 2 does the total number of quality variants correspond to the total number of papers for the following reasons. The 69 variants in Class 2 exceed the 44 Class 2 articles because these studies consider quality in both stages, except for SubCAT 20 articles. The number of variants in Stage 2 of Class 2 thus equals the 44 Class 2 papers. Two CAT2 studies condition the input-output space on quality indicators, in addition to including a priori a single quality indicator in the efficiency model (Gearhart and Michieka 2020) and investigating the association between a specific quality indicator and efficiency in the context of a second stage regression (Ferreira et al. 2013). Since we counted the quality dimensions used to condition the input-output space in both stages, the four CAT2 papers cover eight quality variants. The last line in Table 4 reveals that outcome quality is by far the most frequently included single quality dimension (71). Considering combinations with outcome quality (i.e., SO, PO, SPO), the relevant figure increases to 112. Outcome quality appears as a single dimension in each Class 1 subcategory (46) and more than twice as often in Stage 1 of Class 2 studies (16) compared to Stage 2 (7). S, structural quality; P, process quality; O, outcome quality; SP, structural and process quality; SO, structural and outcome quality; PO, process and outcome quality; SPO, structural, process and outcome quality The studies frequently account for structural quality and combinations thereof in Stage 2. One explanation is that we found mixed handling of some structural quality indicators, such as teaching status. Some authors refer directly to quality when using these indicators, while others do not. To ensure a uniform approach, we classified them as structural quality indicators, regardless of whether the authors made direct reference to structural quality or not. Process quality is predominantly incorporated in Class 1 studies.
Breaking down the subcategories by DMU reveals the following (Table 5). In the country, regional, and hospital studies, outcome quality is the most frequently considered quality dimension. Structural quality is the predominant dimension in nursing and aged care studies, and process quality plays a significant role in primary health care studies. Hospital studies mainly account for outcome quality in the first stage and structural quality in the second stage. Nursing and aged care studies and primary health care studies incorporate outcome quality only in the efficiency model. HC plus NC/AC, cross-provider assessment including hospital and nursing/aged care; HC plus PHC, cross-provider assessment including hospital and primary health care; NC/AC, nursing/aged care; PHC primary health care; OTHER, all other providers

Quality indicators by decision-making unit
This section highlights a few quality indicators, while the Appendix provides a complete list of the structural, process, and outcome indicators used in the reviewed articles.
As the choice of quality indicators, among other things, heavily depends on the DMU category, we describe the indicators by DMU category. Given that outcome quality plays an essential role and is often directly included in the efficiency model, Table 2 also indicates the treatment of undesirable indicators in the efficiency model.

Country studies
Country studies belong to CAT1. Eight of nine country studies incorporate quality directly in the efficiency model (Class 1), while one study uses a two-stage approach (Class 2). Five of eight Class 1 studies are effectiveness studies (SubCAT 13), which means they only include quality outputs. These include, for example, life expectancy, disability-adjusted life expectancy, infant mortality, and potential years of life lost.
One study additionally considers the share of infants immunized against measles as a process quality indicator (Ortega et al. 2017). Santos et al. (2012) investigate the process quality of health systems concerning preventing mother-to-child HIV infection. They use HIV tests for pregnant women up to appropriate therapies with antiretroviral medication for mothers and babies. Kim et al. (2020) use NDEA (SubCAT 15) to break down health system efficiency into health system investment and health system competitiveness. The authors incorporate the life expectancy, under-five mortality, and incidence of tuberculosis cases. Puertas et al. (2020) integrate healthy life years and infant mortality as outcome quality indicators in Stage 1 and investigate the variation in perceived health status using the effectiveness scores as explanatory variables in Stage 2 (SubCAT 23). Therefore, this study accounts for objective outcome quality in Stage 1 and investigates whether differences in subjective outcome quality are related to differences in effectiveness in Stage 2. Two studies assume that efficiency and quality represent different performance dimensions (Lo Storto and Goncharuk 2017; Kočišová et al. 2020) (SubCAT 12).

Regional studies
Effectiveness studies (SubCATs 13 and 23) are pretty widespread in the "HR" category. Class 1 (SubCAT 13) comprises assessments of Italian, Spanish, Mexican, and South African regions (Pelone et al. 2012;Carrillo and Jorge 2017;Serván-Mori et al. 2018;Ngobeni et al. 2020), UK health authorities (Salinas-Jiménez and Smith 1996), and Clinical Commissioning Groups (Takundwa et al. 2017). These studies differ considerably in terms of the considered quality dimensions. Pelone et al. (2012) use the diabetes complication and coronary heart failure (CHF) hospitalization rates as disease-specific undesirable outcome indicators and the vaccination and referral rates as proxies for process quality. Serván-Mori et al. (2018) concentrate on process quality in providing maternal health services at the regional level, using indicators tailored to maternal care. Salinas-Jiménez and Smith (1996) and Takundwa et al. (2017) analyze the effectiveness of UK Health Boards and Clinical Commissioning Groups, respectively, albeit from different perspectives. Salinas-Jiménez and Smith (1996) investigate the structural and process quality of UK Health Boards, and Takundwa et al. (2017) concentrate on the outcome quality of Clinical Commissioning Groups. Typical for structural quality, Salinas-Jiménez and Smith (1996) incorporate characteristics involving General Practitioners (GPs) settings, such as the number of GPs per 10,000 listed patients, the share of GPs with less than 2,500 listed patients, and non-single-handed practicing GPs. They also use common process quality indicators, such as the share of GPs with higher targets for particular services, which also appeared in the country studies. Takundwa et al. (2017) incorporate objective outcome quality measures such as age-and sex-standardized survival rates, in addition to the willingness to recommend a GP practice as a subjective outcome quality indicator. Carrillo and Jorge (2017) also concentrate on objective and subjective outcome quality using healthy life expectancy at birth, infant survival, and self-perceived health status. Overall, the outcome indicators in the country and regional studies are pretty similar.
Five studies are Class 2 effectiveness studies (SubCAT 23), which differ in quality dimensions and indicators. Campanella et al. (2017), evaluating the efficiency of Italian hospital trusts, use disease-specific outcome quality indicators at both stages. While they include disease-specific mortality rates in their efficiency model, they use deficiencies in the outpatient care setting regarding avoidable hospital discharges and complication rates to explain efficiency differences among Italian hospital trusts. De et al. (2012) incorporate child immunization rates as process quality proxies in a second stage to examine the efficiency differences of Indian states. Amado and dos Santos (2009) and Allin et al. (2016) include objective outcome quality indicators in both stages and structural quality as an explanatory variable in Stage 2. Serván-Mori et al.
(2020), in line with a previous study (Serván-Mori et al. 2018), investigate efficiency in providing maternal services at the regional level by including process quality indicators in the efficiency model. In Stage 2, they examine the variation in the maternal mortality ratio in Mexico, considering the efficiency from Stage 1 as an explanatory variable. Common to Class 2 effectiveness studies is that they account for structural quality indicators, if at all, in Stage 2.
SubCAT 11 is also a common subcategory. As regards their structure and process indicators, Giuffrida and Gravelle (2001) strongly rely on the quality indicators used by Salinas-Jiménez and Smith (1996). In addition, however, they also consider the number of patients registered with the GP as quantity output and the standardized mortality ratio as a nondiscretionary factor in the efficiency model to evaluate UK Family Health Authorities. Thanassoulis et al. (1995) were among the first to include both subjective (patient satisfaction) and objective outcome (babies at risk and their survival rates) quality indicators to assess the efficiency of UK District Health Authorities in providing perinatal care. Campoli et al. (2020) use common generic outcome indicators, such as life expectancy, child and maternal mortality, to investigate the efficiency of Brazilian Federative Units. Karsak and Karadayi (2017) use patient assessments of physical hospital characteristics (tangibility) and staff responsiveness (responsiveness) to derive two composite quality indexes. We categorized these proxies as outcome quality indicators (rather than structural and process quality indicators) because they represent patient views. Zheng et al. (2019) investigate the efficiency of the Chinese health care system at the township and village levels. They use the average length of stay as a quality proxy at the township level. We assigned this indicator to structural quality because the authors related quality deficiencies to the lack of licensed doctors and advanced technologies in rural areas. Du (2018), first developing a composite quality index, assesses the effect of quality on the efficiency of Chinese provinces by using models with and without the quality index (SubCAT 14). Since the composite index includes structural, process, and outcome quality elements, we assigned it to all three dimensions. In a similar approach, Polisena et al. (2010) investigate the efficiency of Canadian health integration networks using avoidable hospitalization rates. Gearhart and Michieka (2020) condition the input-output space on quality-related attributes of the health care environment (personnel and facilities) (CAT2). They additionally consider the percentage of non-low weight births and the number of potential years of life gained below 75 years per 100,000 population as outcome quality indicators in the efficiency model. In evaluating the efficiency of Portuguese health regions, Ferreira et al. (2013) use a conditional order-m approach 5 considering mortality rates. In addition, the authors investigate the association between efficiency and the share of patient complaints in the respective health region.

Hospital studies, including cross-provider studies
Hospital studies represent the largest DMU group, with no significant difference in the number of Class 1 and Class 2 articles. Studies in SubCat 11, the dominating subcategory in Class 1, do not incorporate structural quality. However, three studies include only process quality indicators. Miller et al. (2017) use disease-specific process indicators to investigate the effect of a US health care reform on hospital efficiency. Nedelea and Fannin (2017) explore efficiency differences between US hospitals against the background of their different reimbursement schemes. They use disease-specific clinical indicators similar to those of Miller et al. (2017). Matranga and Sapienza (2015) concentrate on organizational processes and use inappropriate discharges as proxies to evaluate the efficiency of Italian hospitals. Azadeh et al. (2016) first use a simulation approach to derive different scenarios for providing neurosurgical intensive care and then apply DEA to identify the most efficient scenario. The authors, using Iranian data, include mortality rates and bedsore patients as outcome quality indicators and average waiting days for operations and average postponed surgery plans as process indicators. Giménez et al. (2019) proxy process quality using the number of days waiting for an appointment and the minutes waiting for treatment in the emergency room. They use subjective (patient satisfaction) and objective indicators (readmissions and referrals) for outcome quality. The other SubCAT 11 studies concentrate on outcome quality indicators, such as mortality (survival), readmission, infection, and referral rates. Park et al. (2018) use DEA twice. First, they derive a composite quality index using data from a patient survey covering satisfaction with medical care and service. Then they assess the efficiency of Korean hospitals using the quality index to adjust profits.
In a comparable study, Ferrier and Trivitt (2013) use DEA first to derive a process quality index, an outcome quality index, and an aggregate process and outcome quality index for selected diseases. Next, they examine separate models and use these indexes to adjust quantity outputs. In contrast to Park et al. (2018), they use various specifications so that their study falls into SubCAT 14. Navarro-Espigares and Torres (2011) apply an efficiency model without quality information, efficiency models with varying objective indicators entailing structural and process quality, and an effectiveness model with perceived quality (patient satisfaction) as a single output. Nayar and Ozcan (2008) incorporate only process quality indicators to analyze the efficiency of US hospitals. The remaining studies in SubCAT 14 include common outcome quality indicators.
With four studies, hospital effectiveness studies (SubCAT 13) are less common. These studies incorporate outcome quality indicators, with the majority relying on objective indicators, such as dead (live) hospital discharges, disease-specific readmission rates, and mortality and infection rates. However, Kholghabad et al. (2019) use patient satisfaction to investigate the effectiveness of Iranian emergency departments. In a single SubCAT 15 study, Cohen-Kadosh and Sinuany-Stern (2020) account for process and outcome quality indicators tailored to assessing the surgical treatment of hip fracture patients. They use NDEA to investigate the efficiency of orthopedic departments in acute care Israeli hospitals.
The SubCATs 20 and 21 studies are relatively balanced. In SubCAT 20, by definition, quality is not considered in Stage 1. With one exception (Dutta et al. 2014), the studies in SubCATs 21 to 25 account for structural quality only in Stage 2. Frequently used structural quality indicators include teaching status, certification and accreditation, 6 technology status, and the availability of qualified personnel (e.g., registered nurses). Some studies tailored the structural quality indicators to the respective research question. For example, Huerta et al. (2011) rely on computerized physician order entries to examine the association between health information technology and US hospital efficiency. Process indicators appear in Stage 2, mainly because we assumed that the composite indexes such as certification and accreditation indexes contain elements of process quality. The outcome quality indicators used in SubCATs 20 and 21 are comparable to those in Class 1. Ferreira et al. (2020), who incorporate composite indexes capturing appropriateness, timeliness, and accessibility of care in a second stage, justify their two-stage approach based on their previous findings that the separability condition holds for Portuguese hospitals (see Ferreira and Marques 2019). 7 Roth et al. (2019), using a two-stage approach, are the only ones who assume that quality and efficiency represent different dimensions of hospital performance (SubCAT 22). SubCAT 24 and SubCAT 25 are rare. Chiu et al. (2008) incorporate structural quality at both stages, using medical equipment and staff as proxies. They also consider the doctor's time spent for diagnosis, categorized as a process quality indicator. Li (2015) examines the relationship between mortality and efficiency using mortality measures from different aggregation levels. Khushalani and Ozcan (2017) 8 use a dynamic NDEA to analyze the evolution of efficiency and quality of US hospitals. They decompose the care process into a surgical/medical subprocess (including only quantity outputs 9 ) and a quality subprocess (including subjective and objective diseasespecific outcome quality indicators).
Two more studies show a connection to the hospital sector (HC plus NC/AC and HC plus PHC). Laine et al. (2005) investigate efficiency in providing long-term nursing care services in hospitals and nursing care facilities. The authors incorporate indicators covering all quality dimensions and consider them to affect the distribution of inefficiency scores (SubCAT 20). Their process and outcome quality indicators differ considerably from those used in hospital efficiency studies, as their focus is on maintaining and restoring the physical and mental health of the elderly rather than providing acute care. However, the authors offer a rich insight into quality indicators suitable for incorporation into nursing care studies. Obure et al. (2016) connect hospital care with that of primary care. The study relies on models with and without quality (SubCAT 14). It includes composite quality indexes that capture structural and process quality elements in assessing the efficiency of various health facilities (e.g., hospitals, clinics, and other health facilities, centers, and units offering services for HIV patients and their sexual and reproductive health).
Two CAT2 studies relate to the hospital sector. Varabyova et al. (2017) use a conditional order-m approach to assess the efficiency of German cardiology departments. They investigate the impact of process quality (patient radiation exposure) and subjective (patient satisfaction), and objective (mortality ratio) outcome quality indicators on the input-output space. Ferreira and Marques (2019) choose a conditional order-α 10 approach to analyze the effect of quality indicators covering elements of all quality dimensions on the efficiency of Portuguese hospitals.

Nursing and aged care studies
Researchers started to look at the quality of nursing care quite early. The first studies date back to the early 1990s. The number of Class 1 and Class 2 studies is balanced. As for effectiveness studies (SubCAT 13), Bott et al. (2007) and Shen et al. (2020) concentrate on organizational processes of US nursing facilities and Korean halfway houses, respectively. Shwartz et al. (2016) and Min et al. (2016) address outcome quality and use care-specific indicators, such as patients with pressure ulcers and other mental and physical restraints. Shwartz et al. (2016) additionally consider risk-adjusted mortality as an outcome quality indicator to analyze the efficiency of US nursing homes. Shimshak and Lenard (2007) use separate models to assess US nursing homes, accounting for outcome quality considering residents with indwelling or external catheters, pressure sores or ulcers, and other restraints (SubCAT 12). Another two studies cover structural quality. Avkiran and McCrystal (2014) incorporate the average length of service of registered nurses in their NDEA approach (SubCAT 15), thereby capturing the quality-related characteristics of the nursing care setting. Kleinsorge and Karney (1992) use a state inspection score and the share of FTEs per resident as indicators of the quality of the care setting (SubCAT 14).
In Class 2, a heterogeneous picture emerges in the corresponding subcategories. Nursing studies consider structural and process quality predominantly in Stage 2 and outcome quality exclusively in Stage 1. Lee

Primary health care studies
Similar to country studies, Class 1 studies dominate in the primary health care category. In SubCAT 11, Aduda et al. (2015) look at the efficiency of providing voluntary medical circumcision for men in Kenyan health facilities using a composite quality index in their efficiency model. Milliken et al. (2011) adjust quantity outputs by using various composite quality indicators in evaluating the efficiency of Canadian primary care practices. Ruiz-Rodriguez et al. (2016) account for process quality in assessing the efficiency of providing services related to women's health prevention and promotion programs in Columbia. Rocha et al. (2014) evaluate management practices in Brazilian family health teams using indicators related to organizational processes (SubCAT 13). In SubCAT 14, Ferrera et al. (2014) rely on a composite quality index with structural and process quality elements to assess the efficiency of primary care centers in Spain. Gouveia et al. (2016) and Razzaq et al. (2013) address patient satisfaction to identify best practices in Portuguese and Iranian primary care centers, respectively.
In contrast to these studies, Glenngård (2013) and McGarvey et al. (2019) treat quality and efficiency as separate performance dimensions (SubCAT 12). The former assesses Swedish primary care centers. The latter focuses on prenatal care provided in US health centers, considering tailored process (percentage of prenatal patients receiving care in the first trimester) and outcome (percentage of low birth weight births) quality indicators. Yuan et al. (2020) rely on composite quality indexes to evaluate Chinese community health centers as proxies for outcome quality within a NDEA framework (SubCAT 15). In a single Class 2 study, Adang et al. (2016) examine the effectiveness of Dutch primary health care in preventing cardiovascular diseases by focusing on cardiovascular risk management (SubCAT 23). They incorporate diseasespecific process and outcome quality indicators in Stage 1 and proxies for structural quality in Stage 2.

Other studies
This collective DMU category comprises physician effectiveness studies (Dowd et al. 2014;Fiallos et al. 2017), insurance efficiency studies (Bryce et al. 2000;Villamil et al. 2017), and medical laboratory efficiency assessments (Ghafari Someh et al. 2020;Lamovšek and Klun 2020). A single article evaluates the quality of life of Diabetes Type-1 patients (Borg et al. 2019). Due to their heterogeneity, we refrain from presenting the studies' quality indicators but refer the reader to the Appendix for further information.

Discussion
The decision to account for specific quality dimensions and indicators in efficiency studies results from weighing potential advantages and disadvantages. We, therefore, assume that the differences in the quality dimensions and indicators explain at least in part the breadth of the methodological approaches. Quentin et al. (2019, p. 40) discuss the pros and cons of the Donabedian quality framework, including the corresponding indicators. They note that structural quality is "easily available, reportable and verifiable". The frequent use of indicators such as teaching, accreditation and certification status, and specific technology and staff ratios in hospital studies supports this statement. Nursing care studies also use equivalent proxies. For example, Kleinsorge and Karney (1992) incorporate a state inspection score in their assessment.
Most studies that incorporate structural quality as a single quality dimension in Stage 2 (Class 2) underpin the assessment that "the link between structures and clinical processes or outcomes is often indirect and dependent on the actions of health care providers" (Quentin et al. 2019, p. 40). Two CAT2 studies investigate this issue. Ferreira and Marques (2019) analyze the impact of access and quality indicators on efficiency, while Gearhart and Michieka (2020) concentrate on some staff categories.
However, the frequent use of structural quality indicators in Stage 2 may result from classifying several indicators as proxies for structural quality that some authors did not explicitly associate with this quality dimension. This finding can also indicate the rather diffuse relationship between structural quality on the one hand and process and outcome quality or efficiency on the other (Quentin et al. 2019, p. 38).
Process indicators are objective indicators that are also easy to survey and interpret. In addition, there is a direct link between process and outcome quality. NDEA models are advanced approaches to investigate this link. Cohen-Kadosh and Sinuany-Stern (2020) incorporate the share of patients with surgery within 48 h and drain usage for one day as process indicators in their NDEA model to evaluate the surgical treatment of hip fracture patients in Israeli orthopedic hospital departments.
The main advantage of process indicators is that compliance with process indicators is easy to check, and the assignment of misconduct to the responsible person is straightforward. Process indicators are used to study organizational and medical aspects of service delivery. Disease-specific measures prevail in the latter. We identified both types in the reviewed papers. Rocha et al. (2014) use the share of scheduled queries among team queries and the percentage of actions developed to investigate management practices in Brazilian family health teams. Amado and Dyson (2009) incorporate patients with a diabetes annual review in the previous 12 months, patients receiving antihypertensive medication, and patients under regular treatment with lipid-lowering medicines to assess the efficiency of primary care practices providing diabetes care.
However, process indicators also have drawbacks. As the name suggests, process indicators relate to single processes. Studies assessing efficiency at a highly aggregated level, such as at the hospital level, cannot include all available process indicators. In this regard, we identified one of the following approaches: a focus on selected services or service providers, a focus on a few selected disease-specific indicators, the use of a composite process index, or a combination thereof. With the percentage of prenatal patients who receive care in the first trimester, McGarvey et al. (2019) concentrate on the prenatal services provided at US federally-qualified health centers. Miller et al. (2017) assess the effect of hospital reform on total hospital costs and quality by focusing on three disease-specific process indicators (heart attack, heart failure, and pneumonia). Ferrera et al. (2014) incorporate a composite quality index to assess the efficiency of Spanish primary care centers. Obure et al. (2016) also rely on a quality index to examine the effectiveness of HIV services and services strengthening patients' sexual and reproductive health.
Additionally, a dimensionality problem may arise with the use of process indicators. Nonparametric efficiency studies in particular often evaluate comparatively few DMUs. However, with many process indicators and few DMUs, it is challenging to identify efficiency differences. Using a few condition-specific process indicators and composite quality indexes are possible alternatives. Neither of the two variants is free from disadvantages. While the concentration on some conditions reflects only a small part of process quality, composite indexes may lead to a loss of information.
Outcome quality and its visibility are becoming increasingly important. Outcome quality is particularly relevant to the patients, as it reflects treatment success and affects their wellbeing. The outcome indicators used in the studies are thoroughly homogeneous within the corresponding DMU category. However, the use of outcome quality indicators entails several particularities.
First, we observed a heterogeneous picture for studies that directly included undesirable outcome indicators in the efficiency model. While some authors treat undesirable outcome indicators as inputs (e.g., Guo et al. 2017), others apply a linear transformation (e.g., Adang et al. 2016), while still others transform them a nonlinear way (e.g., Wang et al. 2018). Assuming weak disposability for undesirable outcome indicators is another possibility (e.g., Prior 2006). Still, others use sophisticated metrics such as directional distance functions or hyperbolic measures, which do not require a transformation of undesirable outputs (e.g., Giménez et al. 2019;Villamil et al. 2017).
Second, the studies use objective or subjective indicators as proxies of outcome quality. However, objective indicators are usually not patient invariant and require risk adjustment to capture heterogeneities in the patient population. Using risk-adjusted mortality for three different conditions, Campanella et al. (2017) do just that in assessing the efficiency of Italian hospital trusts. Subjective outcome measures, such as patient satisfaction, are also increasingly being incorporated in efficiency studies. Relevant studies, among other things, examine the relationship between subjective outcome indicators and efficiency (e.g., Barpanda and Sreekumar 2020).
Outcome quality indicators are by far the most widely used indicators, despite the challenges of correctly establishing a direct and immediate causal relationship between a service provider's actions and the effect on the patient's condition (Quentin et al. 2019, p. 53). In hospital efficiency studies, for example, mortality rates are pretty standard outcome quality indicators. However, several factors beyond the service provider's control can affect hospital mortality, including pre-hospital care, patient characteristics, and other environmental factors. Delays in the treatment effects on the patient's health are also an issue. Studies already account for the patient characteristics, delays in treatment effects, and heterogeneous environments. For example, Ferrier and Trivitt (2013) include condition-specific post-discharge mortality rates in assessing US hospitals, and Ferreira et al. (2020) account for various environmental factors in evaluating the efficiency of Portuguese hospitals.

Summary and limitations
The review shows a variety of methods to integrate quality into efficiency studies. One reason for the diverse approaches is certainly the 40-year observation period. Over the years, scholars continued to introduce sophisticated techniques into the literature. Apart from the considerable variation in DMU-types and their aggregation level, the model heterogeneity also results from the differences in research questions. In contrast to Kohl et al. (2018), who assign DEA hospital studies to four broad research questions, the present review does not cluster by DMU-specific research questions. Further clustering would not have produced meaningful results due to comparatively few studies in some DMU categories combined with quite heterogeneous research questions. The model diversity is undoubtedly also due to country-specific features. The reviewed studies cover a broad spectrum of different health systems-from emerging countries with underdeveloped (public) health systems to highly developed countries with comprehensive public health care, where well-trained medical professionals use advanced medical technology. Accordingly, various specifications emerged. Finally, differences in the availability and accessibility of relevant quality data can also explain variations in model specifications.
Regarding the quality dimensions, the review shows that many studies assume that structural quality affects inefficiency distribution. At the same time, they presume that process and outcome quality predominantly impact the efficiency frontier. These findings, however, are subject to several limitations.
First, the choice of the database and keywords may affect the inclusion of studies in the review. The review does not cover papers published in journals not included in the Science Citation Index Expanded, the Social Sciences Citation Index, or the Emerging Sources Index. It does not cover articles with keywords other than those used for the review either. Second, given the recently introduced conditional approaches, we expect a shift in publications favoring CAT2 in the following years due to the dynamic publication activity. Third, the assignment of quality indicators to quality dimensions entails a considerable degree of subjectivity that also carries over to the conclusions drawn in this review. Consequently, we disclose the assignment of the quality indicators to the quality dimensions in the Appendix.
Overall, the review does not recommend how to proceed if one intends to incorporate quality into efficiency analysis. The review's findings highlight that this would be challenging even within a single DMU type. The decision for a model specification depends on many factors, such as the specific research question, the environmental conditions, and the data availability. Instead, the overview of past efficiency approaches combined with the extensive collection of quality indicators by DMU-specific quality dimension can serve as a starting point for future efficiency studies aimed at integrating quality.
Funding Open access funding provided by University of Graz. No funds, grants, or other support was received.
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