Wear time requirements

In order to investigate the minimum amount of wear-time required for a valid hour, day or 14-day period, intraclass correlation (ICC) analyses were conducted, as ICC is a widely used and accepted means of determining measurement agreement [34]. In each of these experiments, data were deleted incrementally and at random and the ICC was calculated between the partially deleted data and the ‘true’ steps at each increment. An ICC threshold of 0.9 was used as the selection criterion to represent 10% similarity of true values [18]. We first investigated the minimum time required within a single hour with adjustment for wear time, and thus the remaining data was divided by the proportion of the wear time and this adjusted value was used for ICC analyses. In the daily and 14-day analyses, adjustments for wear time were not made. For all analyses, two-way mixed-effects agreement models were used [34] and this was conducted with the ‘icc’ function from the ‘rel’ package in R. Fig 2a demonstrates that if 5 minutes of data are present and scaled to 60 minutes, the ICC threshold of 0.9 is reached. In the daily analysis, the ICC threshold was met at 18–19 hours per day (Fig 2b). It is important to note that our ICC comparisons for each day include non-scaled data despite using scaled data in our algorithm (outlined below). When scaling by the proportion of wear time per day, the number of hours required will be lower. We utilise 18 hours to ensure that true data are available from different parts of the day (i.e. morning, afternoon, evening) and this is a conservative requirement in line with previous research [35]. To establish minimum 14-day requirements, the ICC threshold was met at 3 days (Fig 2c). For the final algorithm, we required 4 days including at least one weekend day as the minimum criteria for inclusion, owing to the potential for differential patterns of physical activity between weekdays and weekend days [36].

Data are presented for scaled minutes per hour (A), for hours per day (B) and for number of days per 14 days (C).