A meta-analysis is a safer starting point than a single study – but it won’t necessarily be more reliable.
A meta-analysis is usually part of a systematic review. It’s a heavy-duty effort, and it’s often described as the ultimate study, outweighing all others. The last word. A single study becomes a puny thing, to be ignored even.
But while combined results can carry a lot of weight, there are 3 main problems with the idea that a meta-analysis always trumps a single study.
Firstly, a systematic review and meta-analysis isn’t a formal experimental study. It’s a non-experimental or descriptive study. There are subjective judgments every step of the way, giving small teams of like-minded people plenty of room to steer in a desired direction if they want to. A bad or patchy meta-analysis might not come to as reliable conclusions as a well-conducted, adequately powered single study.
Secondly, it’s not at all unusual for a meta-analysis to be heavily dominated by a single study. A study by Paul Glasziou and colleagues in 2010 found that even when there were several trials, the most precise one carried on average half the weight of the results – and around 80% of the time the conclusion of the meta-analysis was pretty much the same as that single study. Understanding and discussing that dominant study is critical.
The third problem is so big, it gets the next place on this listicle: a single new study can overturn the results of a meta-analysis.…
A meta-analysis is a snapshot in time – it can even be out-of-date the day it’s published.