The paper evaluates and synthesizes research on the question of how error correction affects learners' ability to write accurately, combining qualitative analysis of the relevant studies with quantitative meta-analysis of their findings. The conclusions are that, based on existing research: (a) the best estimate is that correction has a small negative effect on learners' ability to write accurately, and (b) we can be 95% confident that if it has any actual benefits, they are very small. This analysis is followed by discussion of factors that have probably biased the findings in favor of correction groups, the implication being that the conclusions of the meta-analysis probably underestimate the failure of correction.