Look out for unpublished results of fair comparisons.
Many fair comparisons are never published, and outcomes are sometimes left out. Those that are published are more likely to report favourable results. As a consequence, reliance on published reports alone sometimes results in the beneficial effects of treatments being overestimated and the harmful effects being underestimated.
Biased under-reporting of research is a major problem that is far from being solved. It is scientific and ethical malpractice, and wastes research resources. Selective reporting is an important reason why fair comparisons of treatments should have protocols that are registered and searchable in registries such as clinicaltrials.gov. This way studies can be tracked down even if they are not published.
REMEMBER: Think about whether or not the authors of systematic reviews have considered the risk of biased underreporting of fair comparisons.