66 Chapter 4 International Statistical Classification of Disease and Related Health Problems (ICD-10) [37]. For generalizability, the primary, secondary and tertiary (when available) CAS-diagnoses were recoded to the 22 chapters of the ICD-10 and presented in disease groups. Multimorbidity was defined as having one or more additional diagnosis from a different disease group than the primary diagnosis. The (in)ability to work fulltime is reported by insurance physicians using five categories: 1 = at least 8 h per day; 2 = no more than 8 h per day; 3 = no more than roughly 6 h per day; 4 = no more than roughly 4 h per day; and 5 = no more than 2 h per day. Being able to work eight or more hours per day (categories 1–2) was considered as normal ability to work fulltime, all else (categories 3–5) was considered as an inability to work fulltime. Statistical Methods First, applicants were described on age, gender, educational level, primary disease groups, multimorbidity, and the degree of (in)ability to work fulltime. Second, differences between applicants with normal ability to work fulltime and applicants with inability to work fulltime were compared using t-tests for continuous data and Chi2-tests for categorical and ordinal data. Third, univariable and multivariable logistic regression analyses were performed to study the association of each socio-demographic variable (gender, age, educational level) and disease-related variable (primary disease group and multimorbidity) with the inability to work fulltime (no/yes). Disease group “diseases of the musculoskeletal system and connective tissue” was used as reference category. Fourth, for each of the disease groups population attributable fractions (expressed in percentages) were calculated using Levin’s formula [38, 39] to study the proportional attribution of each disease group to the total number of applicants being assessed with an inability to work fulltime. A high positive percentage for a disease group indicates that the specific disease group has a high attributable fraction to the outcome (being assessed with an inability to work fulltime). A negative percentage indicates a protective fraction to the outcome. Furthermore, univariable and multivariable (adjusted for gender, age, educational level and multimorbidity) logistic regression analyses were performed to study if the primary disease group (no/yes) was associated with the inability to work fulltime, in comparison with all the other applicants in the study sample (not having a disease in that specific disease group as a primary diagnosis). Fifth, multivariable logistic analyses were stratified for each disease group to study the associations between gender, age, educational level and multimorbidity and the inability to work fulltime for each specific disease group. ICD-10 disease groups with a small sample size (n is less than 0.1% of total group) were excluded from the logistic regression analyses.
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