The impact of outcome reporting bias in randomised controlled trials on a cohort of systematic reviews -- Kirkham et al. 340: c365 -- BMJ

"Of the 42 meta-analyses with a statistically significant result only, eight (19%) became non-significant after adjustment for outcome reporting bias and 11 (26%) would have overestimated the treatment effect by 20% or more."

"Conclusions:
Outcome reporting bias is an under-recognised problem that affects the conclusions in a substantial proportion of Cochrane reviews. Individuals conducting systematic reviews need to address explicitly the issue of missing outcome data for their review to be considered a reliable source of evidence...."