Where data is collected at multiple points in time in order to assess a change in knowledge, attitudes, beliefs, or behaviors, the data collection approach must be uniformly applied across all time points. Therefore, it is important to pay attention to the following:
- All rounds of data should be collected in the same way, using the same set of instruments (e.g., questionnaire) at each time point. When data are collected at multiple time points using different procedures or instruments, it becomes impossible to conclude whether any observed differences in outcomes are related to the intervention or the change in data collection procedure or instrument.
- Where interviews, focus groups, or written questionnaires are used, the same respondents should be followed from the first through the final round of data collection.
- If following the same group of respondents is impossible or if repeated cross-sectional survey designs are used, every effort should be made to collect data from a group of respondents who are similar to the original target group on key characteristics (age, gender, race, socio-economic status, location).
Ideally, the same data should also be collected from a control group, who have not been targeted by the intervention but are similar to the intervention group on key characteristics, in order to compare differences in knowledge, attitudes, beliefs, or behaviors over time. Any differences in outcomes over time between the control group and the intervention group can more easily be attributed to exposure to the intervention if these two groups are carefully matched on these key characteristics.