The growing amount of electronic word of mouth (eWOM) has significantly affected the way consumers make purchase decisions. Empirical studies establish an effect of eWOM on sales but disagree on which online platforms, products, and eWOM metrics moderate this effect. The authors conduct a meta-analysis of 1532 effect sizes across 96 studies covering 40 platforms and 26 product categories. On average, eWOM is positively correlated with sales (.091), but its effectiveness differs across platform, product, and metric factors. For example, the effectiveness of eWOM on social media platforms is stronger when eWOM receivers can assess their own similarity to eWOM senders, whereas these homophily details do not influence the effectiveness of eWOM for e-commerce platforms. In addition, eWOM has a stronger effect on sales for tangible goods new to the market, while the product life cycle does not moderate the eWOM effectiveness for services. With respect to the eWOM metrics, eWOM volume has a stronger impact on sales than eWOM valence. In addition, negative eWOM does not always jeopardize sales, but high variability does.Keywords: electronic word of mouth, online reviews, online ratings, online platforms, social media, eWOM metrics, eWOM volume, eWOM valence, eWOM variance, bandwagon effect, persuasion effect, meta-analysis Aloe, Ariel M. and Cristopher G. Thompson (2013), "The Synthesis of Partial Effect Sizes,"