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18.1.1  What is an IPD review?

Individual patient data (IPD) meta-analysis is a specific type of systematic review. Instead of extracting data from study publications, the original research data for each participant in an included study are sought directly from the researchers responsible for that study. These data can then be re-analysed centrally and, if appropriate, combined in meta-analyses. Cochrane reviews can be undertaken as IPD reviews, but IPD reviews usually require dedicated staff and would be difficult to conduct in ‘free time’. The approach requires particular skills and usually takes longer and costs more than a conventional systematic review relying on published or aggregate data. However, IPD reviews offer benefits related particularly to the quality of data and the type of analyses that can be done (Stewart 1995, Stewart 2002). For this reason they are considered to be a ‘gold standard’ of systematic review. This chapter aims to provide an overview of the IPD approach to systematic review, to help authors decide whether collecting IPD might be useful and feasible in their review. It does not provide detailed methodology, and anyone contemplating carrying out their first IPD review should seek appropriate advice and guidance from experienced researchers through the IPD Meta-analysis Methods Group (see Box 18.6.a).