The physical activity of an individual normally changes from one day to another. These day-to-day differences are referred to as the within-individual variation, and can arise from factors such as work/school patterns, religious practices, and seasonal effects such as different holiday times or as part of attending to different subsistence tasks (e.g. agricultural). Variability in physical activity is often considered in terms of activity volume but the behaviour can vary by all its sub-dimensions such as intensity distribution, type or context.
Within-individual variation in physical activity is an assessment challenge because population health science research is generally more interested in the habitual level of activity than the physical activity occurring on any single day. In order to estimate the latent habitual level of activity, sampling designs consisting of monitoring periods with sufficient duration and frequency to account for the within-individual variation are required.
Monitoring period(s)
If an individual participated in exactly the same physical activity every day, then one day of monitoring would be sufficient to observe the habitual level. If the individual only had two different types of day, then two days of monitoring would be required to capture the habitual level using the daily average [1]. However, human behaviour is more complex and no two days are truly the same. The greater the difference between days (within-individual variation), the greater the duration of the monitoring period required to reliably assess the habitual level of activity.
Some questionnaire methods account for within-individual variation by assessing physical activity over the past year or longer. A year of monitoring using diary/log methods or body-worn objective methods would likely impose too great a burden. Furthermore, the monitoring period duration for objective methods follows the rule of diminishing returns, so whilst longer monitoring periods are always desirable from an information perspective, the incremental information becomes smaller and smaller. Determining the minimum monitoring period(s) required to reliably capture the latent habitual physical activity level is therefore an important consideration.
Day-to-day variation tends to be greater in children and 4-9 days of monitoring are common. In adults monitoring periods tend to be shorter, e.g. 3-5 days; however 7 days of monitoring may appear more attractive from a face validity point of view if the population under study follows the cultural practices that have a natural period of a week [2, 3]. If there is likely to be a seasonal effect in activity levels, repeat assessments of activity can be built in to the study design at the individual level or this effect can be taken into account at the point of statistical analysis.
References
- Baranowski T, de Moor C. How many days was that? Intra-individual variability and physical activity assessment. Res Q Exerc Sport. 2000;71(2 Suppl):S74-8.
- Doherty A, Jackson D, Hammerla N, Plötz T, Olivier P, Granat MH, et al. Large scale population assessment of physical activity using wrist worn accelerometers: The UK Biobank study. PLOS ONE. 2017;12(2):e0169649.
- Trost SG, McIver KL, Pate RR. Conducting accelerometer-based activity assessments in field-based research. Med Sci Sports Exerc. 2005;37(11 Suppl):S531-43.