This differs in principle from the Amazon-displayed productaverage star rating (abbreviation: displayed star average), the “Average Customer Review” score that Amazon itself displayed for the book at the time we downloaded the data. One reason for the difference is that Amazon rounds the displayed star average to the nearest half-star (e.g., 3.5 or 4.0) — but for our experiments it is preferable to have a greater degree of resolution. Another possible source of difference is the very small (0.7%) fraction of books, mentioned in Section 2, for which the entire set of reviews could not be obtained via AWS: the displayed star average would be partially based on
reviews that came later than the first 100 and which would thus not be in our dataset. However, the mean absolute difference between the computed star average when rounded to the nearest half-star (0.5 increment) and the displayed star average is only 0.02