Transferring and Retraining Learned Information Filters

Any system that learns how to lter documents will suuer poor performance during an initial training phase. One way of addressing this problem is to exploit lters learned by other users in a collaborative fashion. We investigate \direct transfer" of learned lters in this setting|a limiting case for any collab-orative learning system. We evaluate the stability of several diierent learning methods under direct transfer , and conclude that symbolic learning methods that use negatively correlated features of the data perform poorly in transfer, even when they perform well in more conventional evaluation settings. This eeect is robust: it holds for several learning methods, when a diverse set of users is used in training the classiier, and even when the learned classiiers can be adapted to the new user's distribution. Our experiments give rise to several concrete proposals for improving generalization performance in a collaborative setting, including a beneecial variation on a feature selection method that has been widely used in text categorization.