History of the United States (1918–1945) (especially poor people) and prevent inflation. Tires were the first item to be rationed in January 1942 because supplies of natural rubber were interrupted. Gasoline rationing proved an even better way to allocate scarce rubber. By 1943 one needed government issued ration coupons to purchase typewriters, sugar, gasoline, bicycles, footwear, fuel oil, silk, nylon, coffee, stoves, shoes, meat, cheese, butter, margarine, canned foods, dried fruits, jam, and many other items. Some items—like new automobiles and appliances—were no longer made. The rationing system did not apply to used goods (like clothes or cars). The ration system was complex and confusing, but
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