How many days of pedometer monitoring predict monthly ambulatory activity in adults?

PURPOSE: To determine how many days of pedometer monitoring are necessary to estimate monthly ambulatory activity in adults. METHODS: 212 adults (64% female, age=38.3±13.3 years, BMI=27.9±5.3 kg/m2) wore a pedometer (SW-200) for 28 consecutive days. 76.4% were randomly allocated to a reliability group while the remainder (n = 50) comprised a confirmation group. Mean step counts calculated over the 28-day period served as the criterion. Using the reliability group, intra-class correlations (ICCs) were computed for the entire 4 week period, for 3, 2 and 1 weeks, and for different combinations of any 6, 5, 4, 3 and 2 days. The reliability of the recommended time frame was tested in the confirmation group using regression analysis. RESULTS: In the reliability group, the ICC for any single given day was 0.41. All combinations including 6 days or more had ICCs above 0.80. The inclusion of participant characteristics into a regression, alongside mean steps reported during 1 week of monitoring, failed to strengthen the prediction. When tested in the confirmation group, there was a significant relationship between mean step counts calculated from the first week of monitoring and the criterion (adjusted R2 =0.91, CONCLUSION: It is recommended that researchers collect pedometer data over a 7-day period for a reliable estimate of monthly activity in adults. A 7-day period is recommended, as opposed to 6 days (where ICCs were >0.80) because: 1) step counts are characteristically lower on a Sunday, thus for a reliable estimate of habitual activity, Sunday activity should always be included, and 2) in the event of missing data (1 day), data collected on six days will remain sufficiently reliable to estimate mean monthly activity.