Meta-analyses frequently include old trials that are associated with a larger intervention effect: a meta-epidemiological study

Objective: To assess whether meta-analyses include older randomized controlled trials (RCTs) and whether intervention effect differ between older and recent RCTs.& nbsp;Study Design and Setting: In this meta-epidemiological study of 295 meta-analyses (2940 RCTs) published in 2017-2018, we evaluated the difference in intervention effects between older (i.e., published before 2000) and recent RCTs. We also compared effects by quarters of publication year within each meta-analysis (from quarter 1 including the 25% oldest trials to quarter 4 including the 25% most recent trials). A ratio of odds ratio (ROR) < 1 indicates larger effects in older than recent RCTs.& nbsp;Results: Trials published before 2000 and before 1990 represented 25% and 10% of all trials, respectively. Intervention effects were significantly larger for old than recent RCTs (ROR = 0.92, 95% confidence interval [CI] 0.85-1.00, I-2 = 22%). Compared with the most recent trials (quarter 4), intervention effects were significantly larger for the oldest trials (quarter 1) (ROR = 0.85, 95% CI 0.79-0.92) and for trials in quarter 2 (ROR = 0.89, 95% CI 0.83-0.96) but not for trials in quarter 3 (ROR = 0.98, 95% CI 0.91-1.05).& nbsp;Conclusions: Intervention effects were larger for older than recent RCTs. Meta-analyses including older trials should be interpreted cautiously. (C) 2022 Elsevier Inc. All rights reserved.

Automated summary

The study found that intervention effects were significantly larger for older trials compared to recent trials, indicating caution should be taken when interpreting meta-analyses that include older trials.