BACKGROUND: Event-based Ecological Momentary Assessment (EMA) is a method used to capture an individual's experience of physical activity (PA) in a proximal context, often using smartphones to complete internet-based surveys. This method is contingent on participants' compliance in reporting their momentary states. The purpose of this analysis is to explore the alignment between participants' reported survey time and time-stamped survey meta-data. METHODS: In the parent study, participants completed a pre-activity survey (PreS) and a post-activity survey (PostS) for all purposeful PA performed over 10-weeks. Retroactive surveys (RetroS) could be initiated when momentary reporting was not possible. In addition to reporting total PA duration, PreS and PostS items allowed participants to indicate the timing of their report relative to the start and finish of the PA session (immediately/within 10-min, within 10-30-min, more than 30-min from start/finish). The survey software provided the dat and time of each survey initiation and completion. Basic frequency and descriptive statistics were conducted to summarize the proportion of PreS and PostS reports completed more than 30-min outside the activity session and RetroS. Of the momentary reports, the expected timeframe for reporting (ET; activity duration plus time lags indicated in participant-reported descriptions of PreS and PostS completion) was compared to the meta-data timeframe of PreS to PostS completion to determine degree of alignment. RESULTS: Of the 229 PA bouts reported by 8 participants, 42% were documented using RetroS (mean time lag following PA = 8.61+7.12 hours). Of the 137 sessions documented using PreS and PostS, 24% indicated survey completion more than 30-min outside of the PA session (all specific to PostS; mean time lag = 5.24 +3.04 hours). Alignment between participant-reporting timing and meta-data was noted in 92.3% of the 104 sessions characterized by PreS and PostS completed within 30-min of PA session start and finish. CONCLUSION: In this sample, less than half of the sessions were documented with reasonable proximity. When relying on participant-initiated reports to yield insight into target behavioral events (PA), it may be prudent to examine research results with and without non-momentary data. If impactful, loss of momentary event-contingent PA data should be accounted for in participant recruitment and study duration.

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