Statistical analysis
Most analyses are descriptive and report proportions (with their 95% confidence interval [CI] using Wilson score) percentages, medians (interquartile range, IQR) or means (standard deviation, SD). We performed three prespecified subgroup analyses. First, we compared the eight individual usefulness assessments in studies reported before 2000, between 2000-2009 and between 2010-2019. These time cut-offs were chosen as we expected a clear improvement on at least the transparency features due to a wide range of initiatives that started after the millennium. The trial register clinicaltrials.gov was launched in 2000 and the International Committee of Medical Journal Editors (ICMJE) started demanding registration of trials before submission to ICMJE journals in 2005; icmje.org). The comparative analysis compared the most recent time frame (≧2010) with the time frame before that (<2010) using risk ratios (RRs) with 95% CI. Second, we assessed the impact factor (IF) of journals using the Web of Science Journal Citation Report 2019 and pre-specified a ‘high’ IF at a cut-off of ≧4.0 and ‘low’ IF when <4.(15) This cut-off was chosen to include the top specialty journals in obstetrics and gynaecology in the ‘high’ group, that generally have an IF between 4 and 6 (e.g., Obstetrics & Gynecology, British Journal of Obstetrics and Gynaecology). We expected the most recent RCTs (≧2010) published in high impact journals to perform better in some of the usefulness features compared to those in low impact journals. Third, we stratified usefulness features by whether or not there was a statistically significant result (P<0.05) in (at least one) primary outcome. Finally, we counted for each trial for how many of the 13 usefulness items it scored ‘positive’ to provide an overview on the number of trials meeting at least half (7/13) of the items.