4.1. Strengths and Limitations
REGRESSION, WELLS
This RCT has many strengths. The robust research design employs random assignment, uses a waitlist control group, takes place over two years, and has a substantial sample size, all of which help to ensure strong internal validity. The inclusion of a waitlist control group strengthens the study by allowing for the examination of outcomes at three timepoints prior to the intervention. The trends evident in this group across 18 months (i.e., declining MVPA and slighting increasing sedentary time) prior to the intervention, followed by a directional shift after the intervention, help us to rule-out a variety of threats to internal validity (i.e., alternative explanations), such as local history, selection by maturation, and differential regression to the mean [This study is not without limitation. While the study of childcare centers in low-income communities is critical, such a focus presents a variety of practical research challenges. Enrollment within the childcare center tends to be unstable, teacher retention is often low, and leadership changes are not infrequent, all of which create additional challenges to navigate, including attrition. The differential drop in sample size for the intervention (31%) vs. control (12%) was not ideal, but the similarity in results for the completers only (sensitivity analysis) and full sample analysis do support our findings and alleviate worries about the impact of sample loss on the results. Unfortunately, the data documenting the reasons for children missing follow-up were incomplete, with about 60% of children missing this information; 35% had left the center, and 4% had moved to a different class (not in the study). This study did not include a process analysis to determine whether, and at which centers, the intervention was delivered “as intended.” Moreover, despite the merits of accelerometry, some nuances of PA (i.e., weight-bearing tasks) and activity context are not captured and may require direct observation (e.g., Myers + Wells [