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If data providers have followed the supplied data dictionary closely when preparing their IPD, much of the data harmonisation will have been done already, and minor adjustments may be all that are required. If trial investigators are unable or unwilling to prepare data according to suggested pre‐specified formats, the central research team should accept data in whichever format is most convenient, and recode it as necessary.

Beyond simply aligning trial IPD to the data dictionary, there is also the opportunity to standardise definitions of outcomes or participant‐level variables,^7,43 such as scoring or staging systems. For example, in an IPD meta‐analysis examining the effects of chemotherapy for soft tissue sarcoma,¹⁰² different definitions of histological grade were used in the included trials, but with input from trial investigators, it was possible to translate each of these into a high‐ or low‐grade disease category, allowing exploration of treatment effectiveness according to grade.⁴³ It may also be necessary to construct new standardised variables for use in analyses. For example, in an IPD meta‐analysis of the effects of antenatal diet and physical activity on maternal and foetal outcomes, the research team collected data on each woman’s height, baseline weight and parity, as well as the gestational age at birth and foetal birthweight for each baby. This allowed researchers to generate a standardised meta‐analysis definition of ‘small for gestational age’ (< 10th centile), using a bulk birthweight centile calculator.¹⁰³