Science and Education Publishing American Journal of Sports Science and Medicine 2333-4606 4 4 2016 9 24 Planned Missing Design in a School-Based Physical Activity Intervention for Early Adolescence 83 93 EN Christa L. Lilly Department of Biostatistics, West Virginia University School of Public Health, Morgantown, WV, USA Luciana C. Braga Emily Jones Sean Bulger Kibum Cho Eloise Elliott AJSSM2016441 10.12691/ajssm-4-4-1 2016 4 13 2016 8 16 2016 9 22 Purpose: The purpose of this study is to present a longitudinal planned missing design used to conduct a physical activity intervention in public middle schools (6th - 8th grades) in a rural Appalachian county. Method: Program outcome measures were collected at 13 points, with 33% participant random selection, and complete measures of demographic and anthropometric variables. Results: Of the 4,621 randomly selected participants over the three-year span, missing after random selection was highest for pedometers (74.68%), but also high for PACER (44.47%) and 3DPAR (41.9%). No differences were found on demographic or anthropometric variables, suggesting missing completely at random (MCAR) data on the planned missing data. Participants with missing data after random selection were more likely to be older, male, and in the 8th grade, suggesting missing at random (MAR) data. Conclusions: Results suggest the use of the planned missing design allowed for the feasible evaluation of the intervention using modern missing data analysis to account for MCAR and MAR data.