TY - JOUR T1 - Combining and converting groups when extracting data for meta-analysis JF - BMJ Evidence-Based Medicine JO - BMJ EBM DO - 10.1136/bmjebm-2020-111647 SP - bmjebm-2020-111647 AU - Kathryn S Taylor AU - Kamal R Mahtani AU - Jeffrey K Aronson Y1 - 2021/02/08 UR - http://ebm.bmj.com/content/early/2021/02/08/bmjebm-2020-111647.abstract N2 - It is important that as much evidence as possible is included in the pooled analyses of systematic reviews. Therefore, systematic reviewers may want to analyse outcomes that are not completely specified in the reports of studies that they would like to include. However, using equations and other methods it is sometimes possible to fill in missing data and also check the validity of the reported data. Equations that are particularly useful are those for changing the grouping of patients from that which is reported. The desire to change groups often arises in systematic reviews and meta-analyses of interventions. For example, you may wish to combine the baseline data for two or more groups. Consider a clinical question as an example:What is the effect of inhibiting the renin-angiotensin aldosterone system, compared with placebo, on urinary albumin concentrations in patients with type 2 diabetes?Patients with type 2 diabetes may be subgrouped into those with diabetic nephropathy (microalbuminuric) and those without diabetic nephropathy (normoalbuminuric), such as in a study by Sawaki et al,1 which we shall use as an example. They reported the effects of losartan on urinary albumin, measured by the urinary albumin:creatinine ratio (UACR), expressed as milligram of albumin per gram of creatinine (mg/g). Baseline characteristics were reported by treatment group, and some of the data needed for meta-analysis were reported for the subgroups within each treatment group.When compiling a table of baseline characteristics of the study populations of the included studies in a systematic review, you may find that some studies report baseline data for one or more groups, split by treatment, or by another … ER -