Sample size estimation practices in research protocols submitted to Danish scientific ethics committees.

Background: Sample size in research projects is estimated before initiation of the study to minimise type 1 and type 2 error, while keeping the study's financial cost and subject enrolment to a minimum. This study investigates project-specific factors potentially associated with correct estimation of sample size in study protocols.

Methods: Examination of 189 non-commercially sponsored study protocols (84 randomised controlled trials (RCTs) and 105 non-RCT studies) submitted to the Scientific Ethics Committees of The Capitol Region of Denmark from 2013 to 2015.

Results: 119 (63%) study protocols contained a sample size calculation, with a significantly higher rate of sample size calculations in RCT vs non-RCT study protocols (76% vs. 52%, p < 0.001). Significantly more intervention studies than non-intervention studies (69% vs 52%, p = 0.020), studies including blood samples compared to those without (69% vs. 55%, p = 0.045), studies funded by a foundation donation compared to those with no funding (68% vs. 49%, p = 0.040) performed sample size calculations. Further, increasing number of sick patients enrolled (p = 0.048) and newer studies (p = 0.032) were more likely to include a sample size calculation in the protocol.

Conclusions: Estimation of sample size is more often reported in RCT than non-RCT study protocols. Also, intervention studies, studies funded by a foundation donation, studies including blood samples, studies with a greater amount of sick participants and chronologically newer study protocols more often reported a sample size calculation.