Which of the following factors does NOT need to be standardized across groups in an intervention study?

In intervention studies, standardization is crucial for maintaining the integrity and validity of the research findings across different groups. The application of measures, environmental conditions, and timing of treatment must all be consistent to ensure that any observed effects can be attributed to the intervention itself rather than to variations in how the study was conducted.

Personal preferences of participants, however, do not need to be standardized. Allowing participants to have their own preferences can actually enhance the study's ecological validity, as it better simulates real-world conditions where individuals may have different inclinations regarding certain treatments or interventions. This flexibility may also contribute to a more meaningful understanding of how diverse personal preferences can impact the outcomes of the intervention in practice. Therefore, the correct choice is that personal preferences of participants do not require standardization across groups.