GENERAL DISCUSSION 231 9 Importance of research with heterogeneous groups Starting this thesis, we were warned that our group could be too heterogenic to study. Throughout the thesis, manuscripts were rejected by journals with the comment ‘the group is too heterogenic’. Methodology classes learn you that homogenic groups are ‘better’ for research as traditional statistics are more likely to give good outcomes with homogenic groups then with heterogenic groups. However, in our experience even our qualitative research was rejected by one of the journals, based on the argument of ‘too heterogenic’. Based on our experience with children with mitochondrial disorders, I would question “is this really sound?”. Evidence-based practice (EBP) states that decisions about health care are based on patient values, clinical expertise, and the best available, relevant evidence57. But what is the ‘best’ evidence within heterogeneous groups? The sample of a study is supposed to represent the ‘real target group’, otherwise the study is not external valid58. The target group is usually a diverse society with people varying in age, gender, culture and abilities. Why conduct research on a delimited group that only informs you about a small part of your target group? In addition, is the sample and the target group the only factor that determines homo- or heterogenicity? How about the differences or similarities of the context or the activities? It seems likely that internal validity is preferred above the external validity58-60, and a narrow (disabilities orientated) focus is used in determining homo- or heterogenicity. In the literature it is not clear on what grounds to decide if a group is homogenic or heterogenic. In the first part of this thesis we focused on the experiences of children with their activities; we found that the type of activities performed by children with mitochondrial disorder was not dependent on their functional capacities and did not differ from their typically developing peers5, 61. In the second part of the thesis we focused on measuring the quality of performance of activities. Again, the type of activities performed were not influenced by their functional capacities38, 54. Within this research, children had set their own activity goals fitting with their level of functioning. This individualized approach makes the sample being homo- or heterogenic irrelevant as it focuses on participation were all children with a mitochondrial disorder performing everyday activities fitting with their abilities. Therefore, pursuing a homogeneous sample is not interesting as the homogeneity lies in focusing on the activities and participation of the child. We explored whether a measurement tool (the PRPP-Assessment) is applicable in a, if you focus on the biomedical aspect of the individual, heterogeneous group. The advantage of research in this heterogeneous group is that it shows that it can also be useful for many other individuals. This makes it, in my opinion, a suitable method to conduct research that is externally valid because it is 1) ethical (you do not have to research multiple groups which will result in a higher burden for children
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