S112 Alhassan is now with the Dept of Kinesiology, University of Massachusetts, Amherst, MA 01003- 0258. Sirard is with the Division of Epidemiology and Community Health, University of Minnesota, Minneapolis, MN. Spencer is with the Carolina Population Center, University of North Carolina, Chapel Hill, NC. Varady, and Robinson are with the Stanford Prevention Research Center, and Robinson the Division of General Pediatrics, Stanford University School of Medicine, Stanford, CA 94305-5705. Journal of Physical Activity & Health, 2008, 5, Supp 1, S112-S125 © 2008 Human Kinetics, Inc. Estimating Physical Activity From Incomplete Accelerometer Data in Field Studies Sofiya Alhassan, John R. Sirard, Tirzah R. Spencer, Ann Varady, and Thomas N. Robinson Background: The purpose of this study was to develop a data-driven approach for analyzing incomplete accelerometer data from field-base studies. Methods: Multiple days of accelerometer data from the Stanford Girls health Enrichment Multi-site Studies (N = 294 African American girls) were summed across each minute of each day to produce a composite weekday and weekend day. Composite method estimates of physical activity were compared with those derived from methods typically described in the literature (comparison methods). Results: The composite method retained 99.7% and 100% of participants in weekday and weekend-day analysis, respectively, versus 84.7% to 94.2% and 28.6% to 99.0% for the comparison methods. Average wearing times for the composite method for weekday and weekend day were 99.6% and 98.6%, respectively, 91.7% to 93.9% and 82.3% to 95.4% for the comparison methods. Composite-method physical activity estimates were similar to comparison-methods estimates. Conclusion: The composite method used more available accelerometer data than standard approaches, reducing the need to exclude periods within a day, entire days, and participants from analysis. Keywords: ActiGraph, data reduction, African American, girls Data from the National Longitudinal Study of Adolescent Health indicate that approximately 49% of adolescent African American girls reported 2 or fewer instances of vigorous physical activity compared with 37% for their Caucasian counterparts. 1 Accurate quantification of physical activity is necessary to validly document prevalence, compare groups, assess the associations of physical activity with obesity, assess changes in physical activity, and test interventions to increase