Hacker be mitigated. White hats are usually employed by the target system's owner and are typically paid (sometimes quite well) for their work. Their work is not illegal because it is done with the system owner's consent. Black hats or crackers are hackers with malicious intentions. They often steal, exploit, and sell data, and are usually motivated by personal gain. Their work is usually illegal. A cracker is like a black hat hacker, but is specifically someone who is very skilled and tries via hacking to make profits or to benefit, not just to vandalize. Crackers find exploits for system vulnerabilities
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