Background: Measuring the way people vary across time in meeting recommended levels of physical activity should be a fundamental component of public health surveillance. However, we were unaware of prospective cohort studies that had examined this in a population base using convergent measures. Purpose: We examined agreement between two validated measures used to estimate periodic change in the rate of meeting U.S. Healthy People 2010 guidelines for participation in moderate or vigorous physical activity. Methods: A cohort (N = 497) from a random, multiethnic sample of adults living in Hawaii was assessed every 6-months for 2 years starting spring 2004. Latent transition analysis classified people as meeting or not meeting the guidelines. Intra-class kappa statistics and multinomial logistic regression analysis were used to evaluate agreement. Results: Agreement for classifying stable classes of people who met or did not meet the guideline each time was substantial for vigorous activity (kappa ∼ 0.65-0.70) but fair-to-moderate for moderate activity (kappa ∼ 0.38-0.48). Agreement was poorer for classifying people who transitioned between meeting and not meeting the vigorous guideline (kappa ∼ 0.45) or the moderate guideline (kappa ∼ 0.21-0.29). Conclusions: Rates of meeting the guidelines varied across time and were estimated differently by the two measures, especially for moderate activity. This finding illustrates an understudied problem for public health promotion. Accurate classification of change within people is necessary for determining exposure in outcome studies, personal determinants of sufficient activity, and for evaluating whether interventions are successful in sustaining increases in rates of meeting physical activity guidelines.
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