23 General Introduction 1 looking through the lens that provides a full view. With each chapter that follows, the microscope will be zoomed in step-by-step. The last chapter's perspective will be idiographic: fully zoomed in. In the next paragraphs, I will introduce the lens of each chapter in more detail. In Chapter 2 we look through a wide-angle lens. This nomothetic approach is common practice in intellectual disability research and social sciences in general because it permits doing that which most scientists consider to be the ultimate goal: drawing conclusions that generalize to the population. Typically, the question of whether an intervention program is effective or not is approached with a group-level research design. Essentially, randomized controlled studies ask whether or not some form of treatment elicited behavioral change beyond what may be expected based on chance. Conclusions will be drawn for the group, so recruiting a large sample size is important for power considerations. The sample’s characteristics do need to be representative of the population you wish to make claims about. Change can then be assessed with at least two measurements per person, for example comparing a baseline screening to a follow-up assessment. The most typical design to test the effectiveness of an intervention program is then to compare if two groups differ from each other in their change rate. Figure 2 A hypothetical example of Chapter 2’s lens for studying change Note. A hypothetical example of differences in average change of an intervention group (undashed line) and a control group (dashed line) between a baseline and follow-up screening. Figure 2 illustrates a hypothetical result of a study that compares groups. The two lines summarize how all individuals changed between two time
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