A meta-analysis pools results from multiple studies that asked the same question. The output is two numbers. The first is a single overall estimate of the effect. The second, called heterogeneity, measures how much the included studies disagreed with each other. The second number often matters as much as the first.
What it is
A meta-analysis isn't just more data is better. It is a formal pooling that weights studies by sample size and precision, and that quantifies disagreement between studies (heterogeneity).
A meta-analysis differs from a single study in that it summarizes a literature. It differs from a systematic review in that it adds quantitative synthesis on top of the structured search and appraisal process.
Why it sits at the top of the evidence hierarchy
A well-conducted meta-analysis of randomized trials gives a more precise and stable answer than any single trial. It also makes biases such as publication bias and small-study effects visible rather than hidden.
How it can mislead
Garbage in, garbage out. A meta-analysis of weak studies produces a precise-looking estimate of a weak signal. Publication bias can inflate effect sizes. Different inclusion criteria can give different answers.
Why this matters for reading research on anxiety
Most claims about what works for anxiety draw on meta-analyses. Reading them carefully helps separate strong from weak conclusions even when the headline number sounds similar.
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