[Publication bias in trials other than RCTs]

Publication bias arises from the preferential publication of studies with positive, statistically significant, or particularly strong effects. In meta-analysis on the effectiveness of interventions this may, for example, lead to an overestimation of an effect. In studies on harm, though, wish bias and publication bias frequently take opposing directions. In consequence the overall effect is more difficult to interpret. Generally, non randomized studies might be expected to have a higher potential for publication bias as they have lower publication rates. Specifically, adverse effects are often not reported in a standardized way and tend to be secondary outcomes. Related biases such as positive outcome bias and location bias (problems in locating studies, retrieval bias) are therefore also more likely. As yet, however, there is no empirical proof to support this since research activities on the subject of publication bias is almost exclusively focused on randomized controlled trials.