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When clinicians begin their search for the best available evidence to inform decision-making, they are usually directed to the top of the 'evidence pyramid' to find out whether a systematic review and meta-analysis have been conducted. The Cochrane Library 1 is fast filling with systematic reviews and meta-analyses that aim to answer important clinical questions and provide the most reliable evidence to inform practice and research. So what is meta-analysis and how can it contribute to practice?
What is meta-analysis?
Meta-analysis is a research process used to systematically synthesise or merge the findings of single, independent studies, using statistical methods to calculate an overall or 'absolute' effect. 2 Meta-analysis does not simply pool data from smaller studies to achieve a larger sample size. Analysts use well recognised, systematic methods to account for differences in sample size, variability (heterogeneity) in study approach and findings (treatment effects) and test how sensitive their results are to their own systematic review protocol (study selection and statistical analysis). 2 3
The Five-step process
There is debate about the best practice for meta-analysis, however there are five common steps.
Step 1: the research question
A clinical research question is identified and a hypothesis proposed. The likely clinical significance is explained and the study design and analytical plan are justified.
Step 2: systematic review
A systematic review (SR) is specifically designed to address the research question and conducted to identify all studies considered to be both relevant and of sufficiently good quality to warrant inclusion. Often, only studies published in established journals are identified, but identification of 'unpublished' data is important to avoid 'publication bias' or exclusion of studies with negative findings. 4 Some meta-analyses only consider randomised control trials (RCTs) in the quest for highest quality evidence. Other types of 'experimental' and 'quasi-experimental' studies may be included if...