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Data management is usually done in a number of steps, with queries back and forth to trial investigators or other data contacts, to resolve any issues and ensure accuracy (Figure 4.1), ultimately leading to trial data that are in the best possible shape for the IPD meta‐analysis. Therefore, it is one of the most involved stages of an IPD project, and needs sufficient time and resources. Conducting the individual checking and harmonisation steps, and waiting for responses to queries across multiple trials, can take place over many months. As different members of the research team may handle different trials at different times, keeping track of the process and outputs can be challenging. Producing a detailed plan, and adopting a standardised approach to data checking and harmonisation, will help to ensure that the process is implemented consistently across trials, and between those managing the IPD. For example, using a checklist for all data checkers to follow, together with a common suite of statistical analysis code, can help to ensure and maintain consistency.

Regardless of the extent of checking and data transformation needed, it is always sensible to use formal database or statistical software code to carry out the different steps. The code and the associated outputs help to maintain a detailed log of the checks, and any conversions or modifications to the data, thereby providing a comprehensive and transparent audit trail for each trial. It is also important to record where checks have identified problems, how these were (or were not resolved), and equally to record where no problems were identified.

The information generated for each trial may be held on a number of forms, spreadsheets or as output from statistical software. A summary document is a useful means of bringing together the various elements of checking, querying and decision‐making, and might include hyperlinks to the different outputs, together with correspondence from trial teams. Where resource allows, ideally two individuals would independently check each trial, blinded to the other’s results, and compare and discuss the findings. At the very least, another research team member should review the checking results, and discuss problems arising. Any major or sensitive issues should be raised with senior research team members, prior to any dialogue with the trial investigators.

In the following sub‐sections, we suggest a range of checks for IPD obtained from randomised trials evaluating treatment effects,^7,9,43,101 but most of these are applicable to other types of primary study.