Gathering time-series data of behaviors and psychological variables is important to understand, guide, and evaluate behavior-change campaigns and other change processes. However, repeated measurement can affect the phenomena investigated, particularly frequent face-to-face interviews, which are often the only option in developing countries. This article presents three intervention control studies to investigate this issue. Daily diaries in Cuba did not affect behavior or attitudes for persons with intervention but reduced attitudes for persons without intervention. Reactivity of face-to-face interviews in Bolivia was negligible if applied weekly, but strong if applied twice per week. The article concludes with recommendations for gathering time-series data in developing countries.