Our primary interest is in examining the association between textual variables in user-generated reviews and sales. To maintain consistency with prior work, we also examine the association between average review valence and sales. However, prior work has shown that review valence may be correlated with product-level unobservables that may be correlated with sales. In our setting, though we control for differences in the average quality of products through our fixed effects, it is possible that changes in the popularity of the product over time may be correlated with changes in review valence. Thus, this parameter reflects not only the information content of reviews but also may reflect exogenous shocks that may influence product popularity [5]. Similarly, the variable Number of Reviews will also capture changes in product popularity or perceived product quality over time; thus, β5 may reflect the combined effects of a causal relationship between number of reviews and sales [7] and changes in unobserved book popularity over time.