Like all media use, smartphone use is mostly being measured retrospectively with self-reports. This leads to misjudgments due to subjective aggregations and interpretations that are necessary for providing answers. Tracking is regarded as the most advanced, unbiased, and precise method for observing smartphone use and therefore employed as an alternative. However, it remains unclear whether people possibly alter their behavior because they know that they are being observed, which is called reactivity. In this study, we investigate first, whether smartphone and app use duration and frequency are affected by tracking; second, whether effects vary between app types; and third, how long effects persist. We developed an Android tracking app and conducted an anonymous quasi-experiment with smartphone use data from 25 people over a time span of two weeks. The app gathered not only data that were produced after, but also prior to its installation by accessing an internal log file on the device. The results showed that there was a decline in the average duration of app use sessions within the first seven days of tracking. Instant messaging and social media app use duration show similar patterns. We found no changes in the average frequency of smartphone and app use sessions per day. Overall, reactivity effects due to smartphone use tracking are rather weak, which speaks for the method's validity. We advise future researchers to employ a larger sample and control for external influencing factors so reactivity effects can be identified more reliably.
