title: Variability of total step activity in children with cerebral palsy: influence of definition of a day on participant retention within the study.
abstract: BACKGROUND
  Activity monitoring is important to establish accurate daily physical activity levels in children with cerebral palsy (CP). However, few studies address issues around inclusion or exclusion of step count data; in particular, how a valid day should be defined and what impact different lengths of monitoring have on retention of participant data within a study. This study assessed how different 'valid day' definitions influenced inclusion of participant data in final analyses and the subsequent variability of the data.
  
  
  RESULTS
  Sixty-nine children with CP were fitted with a StepWatch™ Activity Monitor and instructed to wear it for a week. Data analysis used two broad definitions of a day, based on either number of steps in a 24 h monitoring period or the number of hours of recorded activity in a 24 h monitoring period. Eight children either did not use the monitor, or used it for only 1 day. The remaining 61 children provided 2 valid days of monitoring defined as >100 recorded steps per 24 h period and 55 (90 %) completed 2 valid days of monitoring with ≥10 h recorded activity per 24 h period. Performance variability in daily step count was lower across 2 days of monitoring when a valid day was defined as ≥10 h recorded activity per 24 h period (ICC = 0.765) and, higher when the definition >100 recorded steps per 24 h period (ICC = 0.62). Only 46 participants (75 %) completed 5 days of monitoring with >100 recorded steps per 24 h period and only 23 (38 %) achieved 5 days of monitoring with ≥10 h recorded activity per 24 h period. Datasets of participants who functioned at GMFCS level II were differentially excluded when the criteria for inclusion in final analysis was 5 valid days of ≥10 h recorded activity per 24 h period, leaving datasets available for only 8 of 32 participant datasets retained in the study.
  
  
  CONCLUSIONS
  We conclude that changes in definition of a valid day have significant impacts on both inclusion of participant data in final analysis and measured variability of total step count.
authors.affiliation: Department of Surgery, Faculty of Medical and Health Sciences, The University of Auckland, Auckland, New Zealand. n.wilson@auckland.ac.nz.|Paediatric Orthopaedic Department, Auckland City Hospital, Auckland, New Zealand. n.wilson@auckland.ac.nz.
authors.forename: Nichola C
authors.initials: NC
authors.lastname: Wilson
authors.affiliation: Health and Rehabilitation Research Institute, Auckland University of Technology, Auckland, New Zealand.
authors.forename: Suzie
authors.initials: S
authors.lastname: Mudge
authors.affiliation: Department of Surgery, Faculty of Medical and Health Sciences, The University of Auckland, Auckland, New Zealand.|Paediatric Orthopaedic Department, Auckland City Hospital, Auckland, New Zealand.
authors.forename: N Susan
authors.initials: NS
authors.lastname: Stott
mesh_terms: D000293:Adolescent; D002547:Cerebral Palsy / Q000503:physiopathology*; D002648:Child; D003710:Demography; D005260:Female; D006801:Humans; D008297:Male; D008991:Monitoring, Physiologic; D009043:Motor Activity / Q000502:physiology; D016138:Walking / Q000502:physiology*
keywords: Accelerometry; Physical activity; Physical disability; Youth