24 Chapter 1 points: the baseline screening and a follow-up screening two months later. Scores on baseline and follow-up screening are averaged across individuals. Because there are just two measurement occasions, the change trajectory between them is then visualized with a straight line. The undashed line reflects the average change of a group of people who received a specific treatment. Their problem behavior was reduced more than a group of people who did not receive that treatment (the dashed line). In this example, we may conclude the intervention is effective. That conclusion (ideally) generalizes to other people, which implies that the nomothetically obtained knowledge can be used to help others (Hekler et al., 2018). Chapter 2 of this dissertation describes a two-arm intervention trial for a major problem in specialized care facilities, but for which there is a dearth of effective programs: alcohol and drug use. The intervention program that was evaluated in this chapter targets personality traits in relation to substance use, and is called ‘Take it Personal!’. More detail on the program and background about substance use as a problem in care settings for those with mild intellectual disability is provided in the chapter. What is most relevant for now, is that this chapter uses the full-view lens that aims to find out whether Take it Personal! – on average – reduces alcohol and drug use in people with mild intellectual disability. Figure 3 A hypothetical example of Chapter 3's lens for studying change Note. A hypothetical example of how subgroup change, in this example men (blue) and women (pink), may differ from their group average change (black lines). In Chapter 3 the same sample from Chapter 2's Take it Personal! trial is studied, but this time with a different lens. The microscope is zoomed in a bit more. Figure 3 shows what this lens may hypothetically reveal: different
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