229 Pediatric Exercise Science, 2012, 24, 229-245 © 2012 Human Kinetics, Inc. Bringolf-Isler, Kriemler, Grize, and Braun-Fahrländer are with the Swiss Tropical and Public Health Institute and the University of Basel, Basel, Switzerland. Mäder and Ruch are with the Federal Institute of Sport, Magglingen, Switzerland. Measuring and Validating Physical Activity and Sedentary Behavior Comparing a Parental Questionnaire to Accelermeter Data and Diaries Bettina Bringolf-Isler Swiss Tropical and Public Health Institute, University of Basel Urs Mäder and Nicole Ruch Federal Institute of Sport Susi Kriemler, Leticia Grize, and Charlotte Braun-Fahrländer Swiss Tropical and Public Health Institute, University of Basel Accurately measuring children’s physical activity and their sedentary behavior is challenging. The present study compared 189 parental responses to a questionnaire surveying physical activity and sedentary behavior of children aged 6–14 years, to accelerometer outputs and time activity diaries for the same group. Responses were analyzed taking age, sex and maternal education into account. Correlation coeficients between questionnaire reports and accelerometer-based physical activity across all age groups were acceptable (up to r = .55). Yet, adjustment for age markedly attenuated these associations, suggesting concomitant inluences of biological and behavioral processes linked to age. The comparisons of general time indications in the questionnaire with 24h-diary records suggested that parents tended to under- and over-report single activities, possibly due to social desirability. We conclude that physical activity questionnaires need to be designed for speciic age groups and be administered in combination with objective measurements. Background It is widely accepted that regular physical activity is beneicial for children’s health (22), whereas sedentary behavior is a risk factor for chronic diseases (20,27), independent of physical activity levels. However, accurately measuring children’s