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Intraindividual variability and reliability in a 7-day exercise record
T Baranowski1, M Smith, W O Thompson
1Rollins School of Public Health of Emory University, Atlanta, GA, USA. tbaranow@notes.mdacc.tmc.edu@internet
Medicine and Science in Sports and Exercise
|December 10, 1999
Summary
Assessing physical activity requires careful attention to reliability. Two weeks of data collection are needed to achieve a reliability of 0.8 for a 7-day physical activity record.
Area of Science:
- Epidemiology
- Biostatistics
- Physical Activity Research
Background:
- High day-to-day variability in physical activity measures reduces reliability.
- Unreliable measures attenuate correlations with other variables.
- Increased variability necessitates more assessment days for reliable physical activity measurement.
Purpose of the Study:
- To investigate the impact of intraindividual variability on physical activity measurement reliability.
- To determine the number of assessment days required to achieve a desired reliability level.
- To evaluate the assumption of compound symmetry in assessing day-to-day physical activity reliability.
Main Methods:
- A 3-year study involving 165 elementary school teachers recording physical activity daily.
- Physical activity analyzed in minutes, MET minutes, and kcal.
- PROC MIXED in SAS used to control for school clustering effects.
Main Results:
- Compound symmetry assumption was not supported across a 7-day record.
- Weekdays showed intercorrelation, while weekends (Saturday and Sunday) had low correlations.
- Compound symmetry was supported across three weeks of assessment.
Conclusions:
- Achieving a reliability of 0.8 for a 7-day physical activity record requires 2 weeks of assessment.
- Reliability of physical activity measures needs careful consideration.
- Increased assessment points are likely necessary to attain desired reliability levels.