Security hacker another crime such as destroying information contained in that system". These subgroups may also be defined by the legal status of their activities. A white hat hacker breaks security for non-malicious reasons, either to test their own security system, perform penetration tests, or vulnerability assessments for a client - or while working for a security company which makes security software. The term is generally synonymous with ethical hacker, and the EC-Council, among others, have developed certifications, courseware, classes, and online training covering the diverse arena of ethical hacking. A "black hat" hacker is a hacker who "violates computer security for
Item-item collaborative filtering Earlier collaborative filtering systems based on rating similarity between users (known as user-user collaborative filtering) had several problems: Item-item models resolve these problems in systems that have more users than items. Item-item models use rating distributions "per item", not "per user". With more users than items, each item tends to have more ratings than each user, so an item's average rating usually doesn't change quickly. This leads to more stable rating distributions in the model, so the model doesn't have to be rebuilt as often. When users consume and then rate an item, that item's similar items are picked from