Application developers currently have very rudimentary ways to find out how users feel about their apps: Users can leave ratings and reviews. If they use the app that likely means they are likely happy with it, and if they uninstall or delete the app, they are likely unhappy with the app experience. However, as app retention and app engagement become more critical, a number of firms are beginning to create APIs and software development kits aimed at analyzing a user's emotions much more directly. In some cases it involves scanning biological indicators and matching them against a library of others. For others, it's a matter of getting app users to submit more personal information up front. Either way, the developers who capitalize on these kinds of technologies have a much greater chance of adding some "emotional intelligence" into their apps. For more on this topic, check out this FierceDeveloper special report.