Unpublished results of fair comparisons may result in biased estimates of intervention effects.
The results of many fair comparison studies are never fully published. Either the whole study is not published or only some of the results are published. Research findings are more likely to be published if they report favourable results. Maybe these are considered more interesting. As a result, reliance on published reports sometimes leads to the beneficial effects of interventions being overestimated and the adverse 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.
BEWARE of the possibility of biased underreporting of fair comparisons.
REMEMBER to check whether the authors of systematic reviews have included unpublished studies.