22 January 2019 - Questions involving medical therapies are often studied more than once. For example, numerous clinical trials have been conducted comparing opioids with placebos or non-opioid analgesics in the treatment of chronic pain. 

In the 18 December 2018, issue of JAMA, Busse et al. evaluated the evidence on opioid efficacy from 96 randomised clinical trials and, as part of that work, used random-effects meta-analysis to synthesise results from 42 randomised clinical trials on the difference in pain reduction among patients taking opioids vs placebo using a 10 cm visual analog scale (Figure 2 in Busse et al). 

Meta-analysis is the process of quantitatively combining study results into a single summary estimate and is a foundational tool for evidence-based medicine. Random-effects meta-analysis is the most common approach.

Read JAMA Guide to Statistics and Methods