Do sample size calculations in longitudinal orthodontic trials use the advantages of this study design?

To examine whether optimal calculations of the sample size are being used in longitudinal orthodontic trials.

Longitudinal orthodontic trials with a minimum of three time points of outcome assessment published between January 1, 2017, and December 30, 2020, were sourced from a single electronic database. Study characteristics at the level of each trial were undertaken independently and in duplicate. Descriptive statistics and summary values were calculated. Inferential statistics (Fisher's exact test and logistic regression) were applied to detect associations between reporting of a sample size calculation and the study characteristics.

A total of 147 trials were analyzed; 75.5% of these trials reported a sample size calculation with none reporting optimal sample size calculation for longitudinal trials. Most of the longitudinal orthodontic trials did not report the correlation and the number of longitudinal measurements in calculating the sample size. An association between reporting of a sample size calculation (yes or no) and the type of journal (orthodontic and non-orthodontic) was detected with higher odds of reporting a sample size calculation in orthodontic journals than in non-orthodontic journals (3.04; 95% confidence interval, 1.4-6.59; P < .01).

The findings of this study highlighted that the undertaking of optimal sample size calculations in longitudinal orthodontic trials is being underused. Greater awareness of the variables required for undertaking the correct sample size calculation in these trials is required to reduce suboptimal research practices.