Glossary
Meta-analysis.
Direct answer
A meta-analysis combines the numerical results of several studies addressing the same question and produces a single pooled estimate with a margin of uncertainty. Larger and more precise studies carry more weight. Meta-analysis is normally the calculation step inside a systematic review rather than a substitute for one.
Pooling presupposes comparability: mixing studies with different administration schedules, measurement times or outcomes yields a number without clear meaning. Measures of spread between studies show how far the results contradict one another. When that heterogeneity is large, the pooled value behaves more like a question than an answer.
In horses, meta-analysis often founders on the variety of measurement methods: lameness may be scored by eye, captured by sensors or recorded on force plates, and those values cannot simply be added together. Human medicine has agreed scales for many questions, which makes pooling considerably easier there.
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The evidence letter
Once a month: new syntheses, checked studies, and what has changed in the state of the evidence.