What does the term 'attrition' refer to in the context of research studies?

The term 'attrition' in the context of research studies specifically refers to participant dropout over time. This phenomenon is particularly significant in longitudinal studies, where researchers follow the same group of individuals over an extended period. Attrition can affect the validity of a study's findings, as it may lead to a non-representative sample and potentially bias the results if certain types of participants are more likely to drop out than others. Understanding attrition is crucial for researchers to interpret their results accurately and to consider strategies for minimizing its impact, such as increasing participant engagement or using statistical methods to address missing data. In contrast, the other options focus on aspects of study design and execution that do not directly pertain to the concept of participant dropout.