Comparative decision systems and methods are disclosed for gathering and mining data representative of purchase decisions. One disclosed comparative decision system detects when a user is comparing items and provides the user with the ability to create a research note storing comparative information for the alternative items. The system displays information about items according to a variety of factors. The user can customize the factors and enter information for each alternative item according to the various factors. Some information may be pre-populated by the system. The research note may be made visible to other users, and may be suggested to another user based on the note's expected helpfulness and relevance to that user. One disclosed method for mining data stored within research notes identifies which factors are given higher relative priorities by users considering a purchase. Another mining method analyzes the effects of price changes on item popularity.