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Table 12 Caveats for interpreting and understanding subpopulation-specific data

From: An approach to addressing subpopulation considerations in systematic reviews: the experience of reviewers supporting the U.S. Preventive Services Task Force

Availability of subpopulation-specific data Caveats
Presence of subpopulation differences in intervention effects - When interpreting the presence of subgroup or subpopulation-specific findings, recall that evidence is usually observational [7]. Consider methodological heterogeneity, confounding and other sources of bias (e.g., publication, misclassification), magnitude and direction of effect and confidence intervals, and plausibility of causal relationships. Confounding can lead to spurious or misleading subgroup results, particularly when subgroup factors are correlated [61].
- When interpreting reported subgroup effects, beware of false positive effects. If multiple subgroup analyses are conducted, the probability of a false positive finding can be high [55]. Results are more likely to be real if they are based on a priori analyses because these have prior evidence supporting them.
- When claiming an intervention effect in a subgroup, consider whether appropriate methods (e.g., p value adjustment, false discovery rates, Bayesian shrinkage estimates, adjusted confidence intervals, or internal or external validation methods) were used to account for the number of contrasts examined [18].
Absence of subpopulation differences in intervention effects - Subgroup analyses are typically underpowered, thus the risk of false negatives is even higher. One should be aware of the remaining possibility of false negatives in the absence of relative intervention effect differences [59].
- Lack of relative intervention effect differences between subgroups may still result in clinically important variations in absolute benefit due to the impact of differences in baseline risk on absolute intervention effect.
- Lack of difference between subgroups defined on single factors (e.g., age, race/ethnicity) is not sufficient reasoning that subpopulation differences do not exist. Subgroups defined through multivariable risk prediction tools are more likely to be clinically applicable and robust, particularly with larger studies. If a body of evidence has similar multivariable subgroup definitions within studies, pooling can increase power [66].
- Even without heterogeneity of intervention effects, not everyone who receives a “proven” intervention will benefit. (For an intervention with a constant 25% relative risk reduction, one-quarter of expected events will be averted, but 75% of events will still occur despite intervention) [67]. Reminding readers of this fact and emphasizing absolute effects within overall event rates is informative. Further, this approach can help clarify why even modest risk of serious harms may, in the end, exert a strong impact on net benefit calculations for the population as well as for individuals [66].
- When data are not definitive and overall benefits are modest, or overall benefits are moderate but intervention is costly, retaining the possibility of heterogeneity of intervention effects in the absence of evidence may be warranted. Consideration of individualized or targeted intervention approaches may still be applicable for future studies.
- In the absence of compelling evidence, the best estimate is the average intervention effect [40].
Overall - If meta-analyses were conducted, reviewers should consider possible explanations of variations between clinical and statistical heterogeneity.
- Caution is warranted for definitive subgroup conclusions in the absence of patient-level meta-analysis or valid study-level methods and replication (or pooling) of within-study subgroup-specific findings across trials [54].
- Intervention-related risks are substantial (at least for some) and factors that appear to predict increased risk for serious harms can be related to subpopulations. When serious harms are a key issue, consider looking for validated risk prediction tools for serious harms to assist in net-benefit considerations, whether or not reviewed data support subgroup differences [40].
- Data to robustly support subgroup and heterogeneity of intervention evaluations are generally not available given the current state of clinical trial reporting [68]. As a result, predicting individual effects occurs less often, even though it is an area of growing interest as the field of precision medicine develops [18, 69]. Recent recommendations may improve the assessment and reporting of heterogeneity in clinical trials going forward [59].