32 Chapter 2 Our findings indicate that the inability of workers to work fulltime, due to disabling chronic health conditions and/or other causes, should be treated as a complex set of personal and environmental factors, and is variable. This aligns with the conclusions of other research in this area, that the inability to work fulltime has a complex character. This underlines the complexities of work disability assessments in general [13, 14]. Measuring of complex concepts often requires multiple measures and methods. Our results correspond with the ICF-model and the dimensions of the biopsychosocial model currently applied in most work disability settings, and confirm that using solely a medical perspective is too narrow [15]. Operationalization of the concept inability to work fulltime revealed a broad perspective involving the disease, personal factors, and environmental factors, and emphasized the importance of the context of the individual. In this context, his/her career is significantly influenced by societal norms, cultural aspects, and policies regarding accepted norms for working fulltime [16, 17]. Although in this study, we did not further explore this normative aspect, for further assessing and operationalizing inability to work fulltime it is vital to take this aspect into account. For example, in a more individualistic culture [18] like the Netherlands, the government has recently been advised to allow workers autonomy in determining the number of hours they work in order to maintain their work-life balance [19]. The reported measurable indicators of inability to work fulltime can be used as signals to assess the number of hours a person is able to work. They are crucial starting points to help assessors to unravel the question of reduced ability to work fulltime. However, some indicators are not necessarily only preconditions of an inability to work fulltime, as they can also be described as generic and frequently occurring expressions of disease. For example, although fatigue can be caused by a disease or treatment, and be a reason to stop working or reduce tasks, it can also be a response to working too many hours or having too great a workload. Our results are in line with the Dutch guideline [7], which mentions fatigue and cognitive impairments as aspects requiring special attention because they may be related to loss of energy and increased need for recovery. Several validated self-report instruments are available to measure the reported indicators, such as questionnaires on fatigue or energy deficits [20, 21], pain [22], cognitive impairments [23], and functioning [24–26]. Data from these questionnaires can be supplemented with more objective measurements, as from tests like the Psychomotor Vigilance Test [27]; ActiGraph [28]; observation, for example during work and daily life [29] – combined with an assessment interview (as it was mentioned that selfassessment alone was not sufficient, because of the uncertainty regarding the use of this approach alone). This conclusion is in line with that of previous
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