1 Introduction

China’s population has changed greatly since the 1950s. Mortality declined significantly in the 1960s and 1970s and there were rapid changes to fertility after the 1970s. These changes, together, contributed to a relatively rapid process of demographic transition in China. Because the increase in actual numbers of elderly and the increased proportion of the elderly population to total population are both unprecedented, a response to population ageing and the challenges that accompany this ageing has a top priority for the Chinese state. Reliable information and solid evidence from research findings are important for strategy and policy development. There are two principal sources of data for the study of ageing issues and the elderly population. One of these is national-level data collected by the National Bureau of Statistics of China, including population censuses and population sampling surveys. Sampling surveys by research institutions provide the second source of data.

In the 1980s and 1990s, several surveys of elderly people were carried out. For example, a 1‰ sample survey of the elderly population in three provinces and two cities in China was conducted in 1986 by the Population Research Institute at Peking University in association with five other universities (Zhang 1991). In 1987, the Population Research Institute at the Chinese Academy of Social Sciences conducted a sample survey of China’s elderly population aged 60 and above in 28 provinces (Tian 1988), and another sampling survey of the situation of elderly people in nine cities of China was conducted in 1988 by the Institute of Sociology at Tianjin Academy of Social Sciences (Hu 1991). In 1992, a survey of China’s elderly support system in 12 provinces was conducted by the China National Commission on the Elderly and the China Research Center for Ageing. However, until the end of the 1990s, there was limited data available on the health status and related demographic and socio-economic characteristics of the elderly population in China, especially of elderly people over 80 years of age.

In 1998, a team from Peking University launched the first nationwide Chinese Longitudinal Healthy Longevity Survey (CLHLS). To fill gaps in the data and knowledge about China’s oldest old population (people aged 80 and above), the survey began with a focus on this group and issues related to the health of this group. The survey sampling covers 85% of the total population in China, and was carried out in eight survey waves during the years 1998–2018. This review gives a brief overview of changes in the conditions of elderly people and changes to related policies over the two decades when the eight CLHLSs were completed. The review introduces the sampling design of CLHLS, which was set up to include a large cohort of the oldest old, and especially of centennials (aged 100 and above). We list the core information that was collected for the first survey and identify major items that were added in follow-up surveys, including the 2018 survey. Finally, the review summarizes how CLHLS data is used and by what types of publications.

2 The ageing of China’s population and policy changes during the last two decades

China’s population has aged rapidly during the last two decades. The share of the population aged 65 and above increased from 7.0% in 2000 to 8.9% in 2010, and to 11.9% in 2018. The number of people aged 65 and above increased from 88.2 million in 2000 to 118.9 million in 2010, while the number of centennials doubled during this decade. The number of people aged 65 and above is expected to approach 400 million by the middle of twenty-first century. The number of people aged 80 and above and the proportion of this group in the total population is continuing to rise, with the total number expected to exceed 100 million by the middle of the century and reach a peak of 160 million around 2070 (Wang 2019). China has also undergone rapid socio-economic changes over the last half a century, and there have been numerous changes to public policies affecting urban and rural residents. For the elderly, in addition to increased numbers and a larger proportion of the population, there have been changes in the structure of the elderly population, such as in the educational structure and living arrangements of the elderly.

The level of educational attainment plays an important role in determining the health and socio-economic status of an individual, and the elderly are no exception. The overall level of educational attainment of China’s elderly population has changed greatly in recent decades. Before the 1950s, few Chinese residents could go to school and learn to read; rural women in particular had almost no opportunities to go to school and were largely illiterate. Literacy campaigns carried out in the 1950s and 1960s allowed large numbers of urban and rural residents to learn to read, and primary schools gradually became more common in rural areas during the 1960s and 1970s. The development of education during these years has had an important impact on the educational composition of today’s elderly population. Figure 1, which compares the educational composition of the elderly aged 65 and above in 2000 and 2015, shows the changes brought about by the availability of primary education in both urban and rural areas. Elderly women, especially, benefited from this change. In 2000, nearly three quarters of women aged 65 and above had never attended school, while only 21.6% had attended primary school, and even fewer had junior high school or above education levels. By 2015, 43.1% of elderly women had primary school education, 11.6% had attended junior high school, and 6.0% had high school or higher education levels. Changes in the composition of education attainments of the elderly imply changes in employment histories, income, pension and related social welfare benefits for the elderly, as well as changes in intergenerational relationships and living arrangements within individual families. These changes undoubtedly affect the lives and health of the elderly people as they age.

Fig. 1
figure 1

Source Population Census Office of State Council, Department of Population and Social, Science, and Technology, National Bureau of Statistics of China. 2002. Tabulation of 2000 Population Census of China. Beijing, China Statistics Press; Department of Population and Employment, National Bureau of Statistics of China. 2017. Tabulation of 1% Population Survey. Beijing, China Statistics Press

Education composition of population 65+, 2000 and 2015.

The changes of family structure and to the living arrangements of the elderly are directly related to the nature of intergenerational relationships and how elderly people are cared for. Since China’s reforms and opening up began some 40 years ago, Chinese households have changed in a number of ways. Average family size has become smaller and the structure of families simpler. The average age of family members has increased, living arrangement patterns have changed, and a large number of non-traditional families has emerged. The proportion of one to three-person households rose rapidly after 1990, reaching 65% in 2010, and the average size of households shrank to 3.09, more than 30% smaller than in 1982. The proportion of families with at least one elderly member and the proportion of elderly family members in families is on the rise. In 2010, 88.036 million households had elderly family members aged 65 and above, accounting for 21.9% of all households. Almost 30 million households were made up entirely of people aged 65 and above. Among these were two generation elderly households with young old and the oldest old living together, and single generation elderly households in which elderly brothers and sisters lived together. In terms of living arrangements, compared with 1990, the proportion of elderly people living with their children had decreased by 10 percentage points in 2000. In 2010 63.8% of people aged 80 and above lived with their children, a drop of 12 percentage points from 2000 (Peng and Hu 2015).

In addition to the changes in the characteristics of the elderly population and their families, the continuous improvement of social and economic conditions as a result of development and improved social welfare programs have also changed the lives and living conditions of the elderly. Throughout the 1990s and the first years of the twenty-first century, China experienced rapid economic growth, with GDP per capita maintaining double-digit growth for a number of years, rising from less than 2000 RMB in the early 1990s to nearly 50,000 RMB in 2015. China has moved from being a low-income country to a middle-income country. At the same time, China’s social security system, which is closely related to the welfare of the elderly, has also improved. In the 1990s, the State Council issued decisions on basic old-age insurance for enterprise workers and basic medical insurance for urban workers. Local governments have successively established a basic old-age insurance system for employees and a basic medical insurance system for workers, and the income and health care of urban workers after retirement have been basically guaranteed. In 1999, the State Council promulgated regulations on minimum living security for urban residents, and it was implemented in all the cities, ensuring the basic livelihood of the elderly urban poor. In the twenty-first century, social welfare coverage for elderly people has been extended to include rural areas as well as urban areas, and from including salaried workers to including almost everyone. In 2012, the State Council introduced a new type of social old-age insurance in rural areas and social old-age insurance for urban residents, extending basic old-age insurance coverage to the entire country. The new rural cooperative health insurance system began trials in 2003 and scaled up rapidly after 2005. In 2007, pilot programs for the basic health insurance system for urban residents got underway. In 2016, the health insurance system in rural and urban areas was integrated into a basic health insurance system for both urban and rural residents, achieving full coverage nationally. At the same time, there has been a steady improvement to the medical assistance system in both urban and rural areas, giving full play to the supporting function of social assistance (He et al. 2018).

In addition to changes in the elderly population and development of the social welfare system, China’s policies for the elderly have also undergone important changes, and the policy system for the elderly has gradually developed and been improved. In 1999, the State Council established the National Working Commission on Ageing, and in 2001, introduced the first development plan on ageing. The State Council also included sections on the Development of Undertakings on Ageing in the 11th and 12th Five-year Plans, providing top-level guidance for the formulation of systematic policies on ageing. By 2013, China’s ageing policy system had taken shape, covering many aspects of life for the elderly population, such as old-age security, health care for the elderly, services for the elderly, education for the elderly, social participation of the elderly, and efforts designed to ensure a friendly social environment for the elderly. Since 2013, many major policies and systems related to ageing have made breakthroughs. The work on elderly policy is regarded as a systematic project related to the overall social situation. Since 2018, the State Council and various government departments have issued nearly 300 policies related to elderly people and ageing issues, and there are more than 20 special plans for the elderly in the national-level 13th Five-year Plan. The areas covered by the policy system have steadily expanded (Zhu et al. 2018). The establishment and improvement of the policy system for the elderly benefit elderly people, both directly and indirectly, and we can look forward to a corresponding improvement in the quality of life for all segments of the elderly population.

3 The beginning of CLHLS and sampling design of the survey

Initiated in 1997, the Chinese Longitudinal Healthy Longevity Survey was aimed at discovering the social, behavioral, environmental, and genetic factors and their interactions that may influence healthy longevity. It was also intended to provide data for academic research and policy dialogue, and especially to provide information about the oldest old people (aged 80 and above), a sub-group of elderly which had not been adequately represented in previous surveys because of small sample size. The 1998 baseline survey provided a clear picture about China’s oldest old with respect to their living arrangements, socioeconomic status, and health status (Zeng et al. 2002).

The sampling design of CLHLS adopted a multi-stage disproportionate and targeted random sampling method, taking into account the needs for a sample that was representative, methods that allowed for the collection of reliable data, and a fieldwork program that was feasible. As age is a key variable in data analysis, site selection took into consideration the need for reliable and accurate age reporting of the population at the site. Some areas were not included in the survey because of proven significant misreporting of age in population censuses. The first CLHLS conducted in 1998 covered 22 provinces: Liaoning, Jilin, Heilongjiang, Hebei, Beijing, Tianjin, Shanxi, Shaanxi, Shanghai, Jiangsu, Zhejiang, Anhui, Fujian, Jiangxi, Shandong, Henan, Hubei, Hunan, Guangdong, Guangxi, Sichuan and Chongqing. The population of the 22 provinces was 985 million in 1990, about 85% of the total population of China at that time.

Although other social surveys of the elderly population existed at that time, information pertaining to the oldest old group was very limited because the sample size of this group in other surveys was relatively small. This was unfortunate, given the numerous health issues and care needs of this group. However, any sample selection that was proportionate, based on the actual age structure of the population, would be highly concentrated towards relatively younger elderly people and female elderly; the sample of oldest old would be too small for in-depth analysis. To obtain a sample with enough of the oldest old, especially of those over 100 years of age, for meaningful analysis, a targeted and disproportionate sampling method had to be adopted.

The sampling method for the first survey took the following steps: (1) roughly 50% of the counties (or county-level cities or districts) with a total of 631 county-level administrative units in the 22 provinces were randomly selected; (2) all centenarians (people aged 100 or above) living in the selected locations were visited; (3) for each centenarian selected, one 80–89 year-old and one 90–99 year-old resident nearby (that is, living in the same village or on the same street, if there was any, or in the same sampled county or city) were interviewed. The interviewees in step 3 were randomly selected for age and sex according to the survey id code of centenarians. The basic idea of selecting the sample in this way was to ensure that roughly equal numbers of centenarians and men and women in their 80s and 90s were interviewed. While the number of male and female interviewees in age 80s and 90s are also roughly the same by single age. The follow-up surveys revisited elderly people who had participated in the previous survey or family members of the deceased previous respondents. In order to ensure that the total sample size remained roughly the same and that the samples were comparable from survey to survey, elderly people who had died or failed to respond to follow-up surveys were replaced by samples nearby from the same region, and of the same sex and same age. Beginning with the third wave CLHLS in 2002, the sampling was expanded to include elderly people aged 65 and above, making the sample representative of the entire elderly population aged 65 and above. By the fifth wave of CLHLS in 2008, sample sites from the province of Hainan were included in the survey, so the sample of CLHLS has covered 23 provinces out of 31 after 2008. Also beginning with the 2008 CLHLS, eight sites (county or city) with exceptionally high proportions of centenarians were selected to conduct in-depth longevity surveys, using the CLHLS questionnaire with additional in-home basic health exams by medical professionals and the collection of blood/urine samples.

CLHLS continued for 20 years; the eighth and final follow-up survey was completed in 2018. The database now contains approximately 130,000 records of surviving and deceased elderly people aged 65 and above (see Table 1). In addition, 40,800 DNA samples from blood or saliva were collected during the eight waves of CLHLS.

Table 1 Sample size of CLHLS (person)

The over-proportionate sampling of the oldest old and male elderly resulting from the sample design provides a satisfactory sample size for estimating and analyzing the corresponding indicators of some sub-groups. However, if users of the data want to calculate the mean or the distribution of variables to reflect the overall situation of the elderly population, or when making comparisons, the use of weight is recommended (and weights were attached to the survey data provided by the research team).

4 The CLHLS questionnaire and key modifications over time

4.1 The core components of the CLHLS questionnaire

The main contents of the questionnaire used in the first survey remained unchanged in the follow-up surveys, but some items were adjusted or revised to keep up with rapidly changing socio-economic conditions or to satisfy new research demands. The core content of CLHLS constituted the main body of all of the surveys; there were no major changes and adjustments in the follow-ups. There were seven core parts to the questionnaire for people aged 65 and above:

  1. 1.

    Part A—Basic Information. Interviewees were asked their age, sex, ethnic group, place of birth and current living arrangements. If the interviewees lived with family members or other people, information about the gender and age of those they lived with were collected, as well as information about their relationship with the elderly interviewee. Interviewees who lived alone or in a nursing home were asked when they began living alone or in a nursing home.

  2. 2.

    Part B—Self-evaluation. Interviewees assessed their current situation and their personality and emotional characteristics. This part of the interview had to be completed by the elderly person being interviewed. For those who were unable to answer by themselves, there was an “unable to answer” option. In this part of the interview, elderly respondents self-evaluated their quality of life (What do you think of your life now?) and health (What do you think of your own health?), and whether their health had changed in the past year. A series of seven items were used to measure the personality characteristics of the interviewees.

  3. 3.

    Part C—Cognitive Ability. Questions in this part of the interview also had to be completed by the elderly interviewees. Among the questions in this section, respondents were asked what time the interview was taking place, and what month and season it was, the date of the lunar calendar Mid-Autumn Festival, and the name of the district or township where they lived. The ability to react, attention ability and computing ability of the respondents were tested, with interviewees required to retell words (nouns), make calculations, and draw figures shown in the appendix of the questionnaire. Interviewees were asked to repeat the three words mentioned above to test their short-term recall ability. Three questions tested speaking and understanding ability of the elderly interviewees. To test coordination, respondents were asked to fold a piece of paper in half and put it on the ground. Finally, at the end of Part C, interviewers noted whether interviewees had been able to answer all of the questions in Parts B and C, and if not, to note what difficulties the elderly respondents had had.

  4. 4.

    Part D – Lifestyle. In this part, respondents were asked questions about diet, smoking, drinking, exercise, housework and their participation in social activities. There were questions about the types and quantities of staple foods interviewees ate, whether they often ate fresh fruits and vegetables, and whether they often ate meat, aquatic products, eggs, bean products, pickles, sugar, and garlic, and whether they drank tea. Respondents were also asked about the sources of their drinking water when they were children, when they were 60 years old and at the time of the interview. Smoking and drinking questions asked not only about current behavior, but also queried the age at which respondents had started and, if applicable, had stopped smoking or drinking, the amounts they smoked and drank, and the types of alcohol they consumed. Physical exercise questions asked about purposeful fitness activities, and included question about the ages at which physical activity began and, if applicable, stopped. Interviewees were also asked the ages at which they began and stopped being engaged in physical labor. In addition, a series of questions were asked about participation in housework, outdoor activities, gardening and the keeping of pets, reading books and newspapers, raising poultry or livestock, playing cards or mahjong, watching TV and listening to the radio, and participating in organized social activities. Respondents were also asked about the number of tourist trips they had taken in the previous 2 years.

  5. 5.

    Part E—Ability to Perform Daily Activities. The questions included in all surveys are the six items on the ability to take care of daily life (that is, activity of daily living, ADL). Beginning with the fourth survey wave in 2005, if the elderly respondents reported needing help from others, additional questions were asked about when they began needing assistance. After 2002, questions were added to the survey about the 8 items of instrumental activity of daily living (IADL).

  6. 6.

    Part F—Personal Background and Family Structure. Interviewees were asked about how many years of schooling they had, the main types of work they did before age 60, and their principal and other sources of income at the time of the survey. Respondents were asked questions about their current marital status and their marital history, and there were questions about the number of years of school attended and the main job before age 60 of the current (or most recent) spouse. Respondents were asked to identify their primary source of care provider when they were ill. They were asked if they could be treated for serious illness in a timely manner, and why, if treatment was not timely, they could not. There were also questions about whether interviewees had access to timely treatment of illness at age 60. And there was question about who paid for their medical expenses. Interviewees were also asked if they often went hungry in childhood. In addition, information about the gender, age and whether they were still alive was collected about the parents, siblings and children of the elderly interviewees. The final questions in this part asked who, if anyone, the interviewee confided in or could ask for help, and if there were monetary or material exchanges between the interviewee and their children.

  7. 7.

    Part G—Physical Health. Information in this part was provided by medical professionals. There was data for each elderly respondent on eyesight and teeth, blood pressure and heart rate, height and weight, and the ability to move upper limbs and legs. Finally, there was information about any illnesses or symptoms currently afflicting interviewees, and whether they were hospitalized or bedridden.

In the final part of the questionnaire, interviewers recorded their observation of the interview. They indicated whether interviewees could hear the questions clearly, whether they would allow having the physical examination, and whether the age information self-reported by the interviewee was accurate. They also noted if someone answered on behalf of the elderly respondent during the visit, and recorded the relationship of this person to the interviewee.

For those elderly who died during intervals between two surveys, their situation before death was recorded based on the memories of family members or informants close to the deceased. Information was collected on the marital status and living arrangements at the time of death, the number of family members living together and the number of generations living together, the place and cause of death, the main caregivers who assisted with daily life activities before death and the number of days of care provided during the month before death. There was information on whether the deceased was bedridden at the time of death and the number of days before death that he or she was confined to a bed; and whether the elderly can be treated in a timely manner, and the number of serious illnesses and days of illness reported after the previous survey until the time of death. The main sources of income and the average annual income of the family during the year before the elderly person’s death were recorded, as were the actual medical expenses incurred in the year before the death. Information was also collected about the facilities available in the home to the elderly person before death and, if the deceased was a rural resident, whether there was a doctor in the village. The ADL of the deceased at the time of death was recorded, and if ADL limited the number of days of ADL limitation. Information was also collected on the number of days that the deceased was completely dependent on others for care before death, the total cost of care, the cost of direct care in the month before death, and the main payer of the cost of care before death. Finally, information on the activities, and the smoking and drinking habits of the deceased were recorded, and the cognitive ability and pain experienced by the deceased were assessed.

4.2 Major modifications to the CLHLS questionnaire

Since the launch of the first CLHLS survey in 1998, numerous changes to the characteristics, family structure and living arrangements of the elderly in China have taken place. These changes have occurred against a background of rapid social and economic development and improvements to public policies. There is no doubt that the lives and health conditions of China’s elderly have changed, and comparisons of the results of the different waves of CLHLS illustrate these changes. Over the years, changing conditions have created new needs for more information from the survey. In response, CLHLS has not only retained the core components of the questionnaire, but the contents of questionnaire have also been expanded to meet the needs for more information and to satisfy the demands of emerging research initiatives.

When the first CLHLS was completed in 1998, much about the health and family situations, and the socio-economic status of the elderly population, and especially of the oldest old, were unknown or not clear. The results of the survey in 1998 provided a demographic profile of the elderly population that filled many of the gaps in knowledge. As the follow-up surveys developed, more information became available, and in-depth analysis of this information revealed new issues that required exploration. The CLHLS research team organized symposiums after most of the surveys. These were seen as platforms for scholars to exchange information, increase awareness of limitations to the questionnaire, and identify areas of research interest that required more information. Therefore, in addition to retaining the core content, each follow-up survey added and adjusted items in the questionnaire to address ongoing changes to the elderly population, socio-economic conditions and the public policy environment, and to meet the needs of research and decision-making.

In the 2002 and 2005 follow-up surveys, 20 items were added to the questionnaire for the elderly interviewees and 5 items were added to the questionnaire for deceased elderly. The 2008 follow-up survey added two questions to the part about speaking, understanding and self-coordination ability, and the mini-mental state assessment scale (MMS) was modified. In 2008, the survey in longevity areas applied additional health exam form in eight longevity sites and conducted lab tests of some biomarkers using blood or urine samples.

Data collection for the 2011–2012 follow-up survey added 13 new PhenX indicators, and there was a total of 32 items in the questionnaire.Footnote 1

The 2018 survey added data collection on mental health and cognitive ability. In addition, the 2018 survey added questions about the ventilation of indoor living environments and the use of insecticides, air fresheners, and household cleaners in the homes of interviewees.

With regard to data gathered for deceased elderly, beginning in 2011, additional information was collected about the retirement situation of the deceased before death and what pension or old-age insurance benefits they received. Information about family members who had lived with the deceased and housing conditions was collected, as well as information about oral hygiene, dental problems, and hearing loss of the elderly before death. These questions were similar to those added to the questionnaire in 2011 for surviving elderly people aged 65 and over. Because hospice care for the elderly is labor intensive, elderly people may have more than one caregiver, so the 2018 questionnaire added information about the relationships of second and third caregivers to the elderly person and the number of days of care provided. The 2018 questionnaire also added 16 questions (IQCODE) about the cognitive function of the elderly, asking relatives (or other informants) to recall the state of cognitive functions of elderly individuals during the six months immediately prior to their deaths.

5 Data use and publications

Data cleaning and evaluations that included validity and reliability testing and analysis of non-responses and loses to follow-up surveys were performed after each survey. The data proved to be reliable and the quality satisfactory (Zeng et al. 2008). During the last 20 years, a number of interdisciplinary and multi-institute cooperative studies have been carried out using the survey data. Nearly eight hundreds academic articles have been published in Chinese and English, more than one hundred master theses or doctoral dissertations have used the data, and eleven books with findings from the CLHLS have been published in Chinese or English. Several policy briefs and advisory reports have been produced as well.

The use of CLHLS data has increased over time as more waves of survey data have become available. Before 2011, an average of 17 papers per year were using CLHLS data, but during the years 2012–2019 that number jumped to an average of 60 papers per year. Several book-length collections of research findings presented at international seminars and conferences focused on CLHLS data analyses (e.g. Zeng et al. 2008). The significant increase in Chinese language publications suggests there is more research interest and policy discussions related to ageing issues in China.

The publications cover multiple research topics. Figure 2 breaks down by research topics papers published in Chinese and in English in peer reviewed academic journals from 1998 to 2019. Health status and the determinants, measurements of health, longevity and the risk of death, and health related biologic, genetic and environmental factors are the most frequently studied topics. Issues related to families, such as living arrangement, intergenerational relationships, and the relationship the family situation has to the health status of elderly people are also frequently studied topics. Studies related to the care of the elderly are, for the most part, published in Chinese language journals, indicating that both the society and academia are paying more attention to this issue in response to the rapid ageing of China’s population. Topics related to health care, medical insurance, and social support also appear mainly in Chinese language publications, a response to the changing situation for health insurance and social welfare programs mentioned earlier. English language studies are often related to psychological and cognitive health, as well as to lifestyle and health.

Fig. 2
figure 2

Number of papers using CLHLS data published by research topics, 1998–2019

Earlier publications were concerned primarily with the general status of the oldest old, and provided demographic and socioeconomic profiles of this sub-group that helped to fill gaps in the knowledge of the oldest old. Health status and the determinants, especially those factors related to healthy longevity, have always been topics of research interest. Because more CLHLS data became available with each follow-up study and the types of data collected expanded over the years, studies using the data have been able to address virtually every dimension of health status and the determinants of that status for elderly people. Over time increasingly sophisticated methodologies have been applied to explore complex relationships in-depth. In the last decade numerous research findings about the socio-economic status and social welfare of elderly people have been published, and more papers than before have been published about psychological health, lifestyle and health, and the relationship of bio-indicators and environmental factors to the health of elderly people. In recent years, a number of papers have focused on the interrelationships of biomarkers, socio-economic and environmental factors as they affect the mortality, disability, and cognition of elderly people. Several papers have examined issues related to the quality of life before death. Longitudinal studies utilizing information from multiple CLHLS waves have also become more common in recent years.

The CLHLS dataset (1998–2014) is available in the Peking University Open Access Research Database (http://opendata.pku.edu.cn) and the National Archive of Computerized Data on Ageing, Inter-university Consortium for Political and Social Research (https://www.icpsr.umich.edu/icpsrweb/NACDA/series/487). When using CLHLS data, please note accompanying explanations describing the sample for each wave and adjustments made to each of the survey questionnaires in order to find the data content corresponding to the appropriate survey year and to meet the needs of specific research topics.