162 Chapter 8 information for the conceptualization and operationalization of our subject. Nevertheless, the participating physicians mentioned in Chapters 2 and 3 may be regarded as experts in their field. They provided us with extensive information about, and insight into, the concept of inability to work fulltime. To represent the patient perspective, we included experienced staff members of organizations representing patients with specific chronic diseases that are common among long-term sick listed workers receiving work disability benefits. We hereby endeavored to acquire the broadest possible picture of inability to work fulltime. Some patient representatives had themselves been patients, while others, themselves not former patients, had to rely on information from patients who were members of their organization. Again, this may have impacted our results, as experiences from patients who had actually experienced inability to work fulltime and who had undergone an assessment, could have provided additional information. Quality of the data The register data used, were derived from assessments conducted by trained assessors, in our case insurance physicians in public practice; they include data from all comprehensive assessments of sick-listed workers applying for work disability benefits in the Netherlands in 2016. A major strength of these data is that all assessors were required to adhere to an existing professional guideline on assessing inability to work fulltime; this enhanced the reliability and validity of the data. Although previous studies revealed the presence of inter-rater variations in the assessment of inability to work fulltime [12, 13], the large sample size, n= 40263, may level off these differences. A disadvantage of register data is that, because they were not originally collected for research purposes, they do not contain all relevant factors and determinants related to inability to work fulltime. For example, data regarding well-known factors related to work disability were not included -- such as severity and treatment of disease; work characteristics; personal factors like coping, motivation and illness perceptions; and time-related aspects such as training or recovery. In addition, we encountered several missing values on the education variable. Upon inquiry we discovered that current internal procedures do not always require the labor expert to include education in the register; this explains the missing values. For some cases, however, we found earlier entries regarding educational level, and used these in the analyses. In the register data, diagnoses are recorded using the Dutch Classification of Occupational Health and Social Insurance, (CAS). For generalizability and international comparison, we recoded the CAS-diagnoses into the 22 chapters of the ICD10 [14]. This allowed us to compare our findings on inability to work fulltime with other studies on, for example, work disability, using similar disease groups [15, 16]. As these disease groups include numerous diagnosis
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