The WHO has recently debated whether to reaffirm its long-standing recommendation of mass drug administration (MDA) in areas with more than 20 percent prevalence of soil-transmitted helminths (hookworm, whipworm, and roundworm). There is consensus that the relevant deworming drugs are safe and effective, so the key question facing policymakers is whether the expected benefits of MDA exceed the roughly $0.30 per treatment cost. The literature on long run educational and economic impacts of deworming suggests that this is the case. However, a recent meta-analysis by Taylor-Robinson et al. (2015), (hereafter TMSDG), disputes these findings. The authors conclude that while treatment of children known to be infected increases weight by 0.75 kg (95 percent CI: 0.24, 1.26; p=0.0038), there is substantial evidence that MDA has no impact on weight or other child outcomes. This paper updates the TMSDG analysis by including studies omitted from that analysis and extracting additional data from included studies, and finds that the TMSDG analysis is underpowered: Power is inadequate to rule out weight gain effects that would make MDA cost effective relative to comparable interventions in similar populations, and underpowered to reject the hypothesis that the effect of MDA is different from the effect that might expected, given deworming's effects on those known to be infected. The hypothesis of a common zero effect of multiple-dose MDA deworming on child weight at longest follow-up is rejected at the 10 percent level using the TMSDG dataset, and with a p value < 0.001 using the updated sample. In the full sample, including studies in settings where prevalence is low enough that the WHO does not recommend deworming, the average effect on child weight is 0.134 kg (95 percent CI: 0.031, 0.236, random effects). In environments with greater than 20 percent prevalence, where the WHO recommends mass treatment, the average effect on child weight is 0.148 kg (95 percent CI: 0.039, 0.258). The implied average effect of MDA on infected children in the full sample is 0.301 kg. At 0.22 kg per U.S. dollar, the estimated average weight gain per dollar is more than 35 times that from school feeding programs as estimated in RCTs. Under-powered meta-analyses are common in health research, and this methodological issue will be increasingly important as growing numbers of economists and other social scientists conduct meta-analysis
