We propose a Bayesian analysis of mediation and moderation effects embedded within a hierarchical structure to examine the impacts of two sources of WOM information — online user reviews and professional reviews in the context of software download. Our empirical results indicate that the impact of user reviews on software download varies over time and such variation is moderated by product variety. The increase in product variety strengthens the impact of positive user reviews, while weakening the impact of negative user reviews. Furthermore, professional reviews influence software download both directly and indirectly, partially mediated by volume of online user reviews. Receiving positive professional reviews leads to more software download, yet receiving very negative professional reviews has a negative impact on the number of download. The increase in professional ratings not only directly promotes software download but also leads to more active user WOM interactions, which in turn leads to more download.