What could be a cause of missing data in research?

The correct choice identifies that one of the common causes of missing data in research is participants dropping out of the study. When participants exit a study before its completion, the data that would have been collected from those individuals becomes unavailable, leading to gaps in the dataset. This dropout can occur for various reasons, such as changes in personal circumstances, loss of interest, or adverse events, and is especially problematic in longitudinal studies where data over time is crucial for analysis.

Other potential causes of missing data include participant non-responses to specific questions or items due to discomfort or confusion, but the dropout scenario is particularly significant because it can impact the overall validity and reliability of the study's outcomes. Addressing this issue is critical in research design, as retaining a consistent participant group helps ensure data integrity.