A community sample of 1231 participants was recruited for the purpose of this study, which included a national online platform of panel members. This platform invites people to register as a panel member and take part in online research in exchange for a financial reward. Participation for the remaining individuals was voluntary and unpaid. Participants were selected to obtain a sample evenly divided in gender and in an age ranging from 18 to 65 years. None of the individuals in the community sample reported to have been diagnosed with ADHD. Sample characteristics, including its subsamples, are presented in Table 1.
Community sample (total sample)
The total sample was used to estimate the prevalence rates of work-related problems and ADHD symptoms in the community.
Community sample (ADHD symptoms)
A subsample of the community sample was selected with elevated levels of ADHD symptoms. The CAARS ADHD index was employed for the selection of participants, i.e., to identify those with a score equal or higher than one standard deviation above the mean (T score ≥ 60; n = 66; Table 1).
Community sample (community comparison group)
A comparison group to patients with ADHD was selected from the community sample which roughly matches the ADHD group (total patient sample; n = 134) in age, gender, and education years. For each patient with ADHD, two individuals from the community sample were selected with the same or similar characteristics in the three variables of interest (age, gender, education years), yielding a community comparison group of 268 individuals. The community comparison group did not differ significantly from patients with ADHD in age, gender or education (Table 2).
Patients with ADHD
One hundred and thirty-four individuals diagnosed with ADHD were selected for participation in this study. All individuals were referred by general practitioners, neurologists, psychiatrists, or self-referred for a diagnostic assessment to the ADHD outpatient clinic of the Department of Psychiatry and Psychotherapy, SRH Hospital Karlsbad-Langensteinbach, Germany. All individuals underwent a comprehensive diagnostic assessment by trained psychologists or psychiatrists. The diagnosis of ADHD was established based on the criteria as outlined in the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM–5; American Psychiatric Association, 2013). The assessment procedure included a semi-structured interview to evaluate ADHD psychopathology (i.e., the Wender–Reimherr Interview; Rösler et al. 2008). Furthermore, a number of self-report scales were completed by all participants to quantify the retrospective and current ADHD symptom severity and psychopathology. Diagnostic veracity was corroborated by the identification of objective evidence of impairment (e.g., financial problems, failure in academic setting, risk behavior, drug use, etc.) and the consult of collateral information (e.g., employer evaluation, partner or parent-reports) whenever possible. The characteristics of patients with ADHD, including clinical information such as symptom presentation, comorbidity, and medication status, are presented in Table 2.
Subsample of patients with ADHD (administering neuropsychological tests)
A subsample of the group of patients with ADHD (n = 51) underwent cognitive testing using a neuropsychological battery. This subsample had a mean age of 36.2 years (11.4 years SD), contained 21 females, and averaged to 12.2 years of education (2.3 years SD). Most of the patients with ADHD from this sample were diagnosed with the combined (n = 28) or inattentive (n = 21) symptom presentation. One patient was diagnosed with hyperactive-impulsive presentation, while the symptom presentation was not reported for another patient.
Questionnaire on individual characteristics
A questionnaire was composed for the purpose of this study asking for individual characteristics such as age, gender, educational attainment, civil status, living situation, and work status. Clinical information from patients with ADHD were obtained from clinical records, the clinical interview, as well as self-report questionnaires.
Conners’ Adult ADHD Rating Scale (CAARS)
The Conners’ Adult ADHD Rating Scale (CAARS; self-report; long version; Conners et al. 1998) is a 66-item inventory that addresses self-reported ADHD symptoms. Answers are scored on a four-point scale (0 = not at all, never; 1 = just a little, once in a while; 2 = pretty much, often; 3 = very much, very frequently). Scores on individual items are summed up yielding eight different scales, with some items contributing to more than one scale. In the present study scales scores for Inattention, Hyperactivity, Impulsivity, and the ADHD index, were used.
Weiss Functional Impairment Rating Scale (WFIRS)
The Weiss Functional Impairment Rating Scale (WFIRS) is a self-report measure for impairments that commonly occur in patients with ADHD and that are likely to represent the patients' targets of treatment (CADDRA, 2017). The WFIRS comprises 70 items that are divided into seven domains: Family (8 items), Work (11 items), School (11 items), Life Skills (12 items), Self-concept (5 items), Social (9 items), and Risk (14 items). Each item is scored on a four-point Likert scale scored from 0 to 3 (0 = never, not at all; 1 = sometimes, somewhat; 2 = often, much; 3 = very often, very much). An additional answering option is given with Not Applicable. A scale score per domain is calculated by summing up the responses to all items per domain (response values 0–3), and dividing this sum by the number of endorsed items (thereby not considering items that are answered with Not Applicable). Furthermore, any item score ≥ 2 was scored to indicate impaired functioning (CADDRA 2017). The WFIRS was reported to have high internal consistency with Cronbach’s alpha > 0.8 for each domain and the scale as a whole. For the purpose of the present study, only the Work subscale will be considered, including both individual item scores (scores ≥ 2) and the Work mean score. Table 3 presents the eleven items comprising the Work subscale of the WFIRS.
Neuropsychological performance tests (subsample of patients with ADHD)
A number of neuropsychological tests were performed to assess aspects of attention and executive control, i.e., selective attention (Perception and Attention Functions—Selective attention, WAFS), vigilance (Perception and Attention Functions—Vigilance, WAFV), cognitive flexibility (Trail Making Test, TMT), verbal fluency (Regensburg Word Fluency, RWT), working memory (N-back Verbal, NBV), and planning (Tower of London—Freiburg version, TOL-F). The tests WAFS, WAFV, TMT, NBV, and TOL-F were taken from test battery Cognitive Functions ADHD (CFADHD; Tucha et al. 2013; Schuhfried 2010). The CFADHD is a computerized test battery assessing cognitive functions in which adults with ADHD commonly experience difficulties. The RWT is a test administered via paper-and-pencil (Aschenbrenner et al. 2000). The administration of the total test battery took about 60 min. Test variables assessing speed (mean reaction time), variability (SD of reaction time), and accuracy (e.g., errors of omissions, commissions, or correct responses) were derived (see Table 6 for on overview of all test variables derived from this battery).
Participants of the community sample took part in the study online. All participants provided active informed consent by clicking the option in the online form that they agreed with participation in this study. Participants were first requested to complete the questionnaire asking for personal characteristics, followed by the CAARS and WFIRS, taking about 15–20 min in total. Only participants that completed the survey were considered for data analysis. The study was approved by the Ethical Committee Psychology (ECP) affiliated with the University of Groningen, The Netherlands.
Patients with ADHD
Patients with ADHD gave written informed consent and completed the survey as well as the test battery, if applicable, as part of a larger research project. Participation in this project was voluntary, unpaid, and was separated from their diagnostic assessment and treatment. All patients with ADHD were asked to complete the set of questionnaires to the best of their knowledge and not to seek help from the examiner or to discuss questions or their responses. A subsample of patients with ADHD (n = 51) additionally performed the neuropsychological battery using cognitive tests. Tests were administered in an office of the psychiatric hospital without distraction. Test administration was led by a trained psychologist and took about 60 min. The study involving patients was approved by the medical ethical committee affiliated to the University of Heidelberg, Germany.
Data were presented by group in descriptive statistics, i.e., giving mean scores and standard deviations (for continuous data) and frequencies (for categorical data). Responses of patients with ADHD and the community comparison group were compared using t-tests for independent samples (continuous data) and Chi-Square tests (categorical data). Associations between ADHD symptom severity and WFIRS Work scores were explored in bivariate correlation analyses. The association between neuropsychological test performance of the subsample of patients with ADHD and work-related problems was explored in multiple linear regression analysis with the neuropsychological test scores as predictors and the WFIRS Work score as the criterion. Furthermore, the association between each of the test variables and the WFIRS Work score was examined in bivariate correlation analyses. Significance level was adjusted to 0.01 to control for alpha error inflation in multiple testing.