Abstract
Reactivity to physical activity (PA) measurement may result from the introduction of a measurement device, researcher observation, or both. Accessing data from prior to study enrollment afforded the rare opportunity to compare behavior during versus prior to participation. This study introduced researcher observation among adults who owned their own PA monitoring device, to test whether measurement reactivity can also be observed in experienced PA trackers, by comparing their data from before versus after the introduction of observation. In addition, the salience of researcher observation was manipulated to test for potential effects. Participants were 252 adults in the U.S. They completed two electronic surveys 14 days apart, in which they recorded steps per day as collected by their PA monitors over the previous 14 days. At the end of the first survey, they were randomized to view messages, which differed in emphasis on repeating entry of step data (i.e., "low" vs. "high" salience of researcher observation). Daily step counts did not change between 14-day reporting periods, though patterns differed by gender and starting level of PA. Patterns did not differ between experimental conditions. Overall, introducing researcher observation without introducing an unfamiliar measurement device results in no meaningful reactivity with respect to PA.