Trial sample size, but not trial quality, is associated with positive study outcome

To assess whether the reported trial quality or trial characteristics are associated with the trial outcome.

We identified all eligible randomized controlled trials (RCTs) of arthroplasty from 1997 and 2006. Trials were classified based on whether the main trial outcome was reported to be positive (n = 90) or negative (n = 94). Multivariable logistic regression analyses studied the association of reporting of trial-quality measures (blinding, placebo use, allocation procedure, overall quality) and trial characteristics (intervention type, number of patients/centers, funding) with positive trial outcome.

RCTs that used placebo or blinded care providers, used pharmacological interventions, had higher Jadad quality scores or sample size of more than 100 patients were significantly more likely to report positive result in univariate analyses. Multivariable regression did not identify methodological quality of RCTs, but rather found that sample size was associated with trial outcome. Studies with more than 100 patients were 2.2 times more likely to report a positive result than smaller studies (P = 0.04).

Lack of association of reported trial quality with positive outcome in multivariable analyses suggests that previously observed association of reported study quality with study outcome in univariate analyses may be mediated by other study characteristics, such as study sample size.