Table 2 Age-treatment subgroup analyses with overlapping subgroup levels

There were 9 reviews with 32 individual age-treatment subgroup analyses based on potentially overlapping subgroup levels (e.g., mean age < 50 years, mean age 50 to < 65 years, vs. mean age 65+ years). The majority (25 of 32, 78.1%) of these subgroup analyses were in reviews that specified plans to conduct subgroup analyses in their methods section. Almost two thirds (20 of 32, 62.5%) of the analyses reported a P value from an interaction test, of which four (4 of 20, 20.0%) were statistically significant. Standardization by effect measures (mean difference or risk ratio) and a random effects model did not change the number of statistically significant age-treatment interactions; however, using a fixed effects model resulted in three additional statistically significant age-treatment interactions. Among the 12 age-treatment subgroup analyses without a reported P value from an interaction test, five analyses were from three reviews that outlined plans to conduct an age-treatment subgroup interaction test.