If nothing happens, is everything all right? Distinguishing genuine reassurance from a false sense of security

Safety of transcranial direct current stimulation: Evidence based update 2016

“…Our results are consistent with another review showing that 40.7% of the studies assessing treatment with tDCS in stroke patients fail to mention whether adverse effects were measured or not and whether they occurred or not (11). Therefore, the conclusion that there are no reports of serious adverse effects associated to tDCS could better represent an "absence of evidence of harms" rather than "evidence of absence of harms" (12).…”

PRISMA harms checklist: improving harms reporting in systematic reviews

“…55-57 An analysis of systematic reviews from the Cochrane Collaboration found that the single primary harm outcome was inadequately reported in 76% of the studies included and not reported in 47% of reviews outside Cochrane that have been designed specifically to measure harms. 56 A similar study of reviews of efficacy found that 31% of trials did not report on the primary benefit outcome. 57 There is also evidence that those reporting on primary studies might choose to play down the estimates of harms and emphasise the efficacy of the intervention instead.…”

Zero is an important number

“…Pooling studies that are confounded produces an overall summary estimate that also is confounded. Zedler et al would have provided a more valid effect estimate had they attempted to remove some of the uncontrolled confounding from their pooled estimate, perhaps by bias analysis simulation [14], as was performed in our meta-analysis published in 2014 [15]. We showed that confounding in published cohort studies and confounding and/or selection bias in randomized controlled trials from study dropout could contribute to the observed protective effect of buprenorphine versus methadone on the neonate.…”

“…This is a problem for systematic reviews, particularly those focused on assessing harm. Emerging evidence shows that the risk of reporting bias in primary studies is greater for adverse outcomes than for efficacy outcomes [13][14][15].…”

“…Studies with zero events still contribute important information, and excluding studies on the basis of no adverse events leads potentially to overestimation of harm and a biased review. If studies are excluded on the basis that zero events do not contribute meaningful data to a review, then it is important for review authors to clarify if there was an absence of events or an absence of reporting of adverse events for all potentially relevant studies [14]. Otherwise, reviewers risk exacerbating the bias of the primary studies if they also fail to report fully on harms.…”