. Mặc dù nó không rõ ràng khi có năm mặt hàng, tăng số lượng các mục trong các kết hợp đòi hỏi tion computa theo cấp số nhân hơn. Kết quả theo cấp số nhân tăng trưởng lần và lâu dài, dài chờ đợi khi xem xét kết hợp với hơn ba hoặc bốn mặt hàng. | 312 Chapter 9 For instance in the grocery store that sells orange juice milk detergent soda and window cleaner the first step calculates the counts for each of these items. During the second step the following counts are created Milk and detergent milk and soda milk and cleaner Detergent and soda detergent and cleaner Soda and cleaner This is a total of 10 pairs of items. The third pass takes all combinations of three items and so on. Of course each of these stages may require a separate pass through the data or multiple stages can be combined into a single pass by considering different numbers of combinations at the same time. Although it is not obvious when there are just five items increasing the number of items in the combinations requires exponentially more computation. This results in exponentially growing run times and long long waits when considering combinations with more than three or four items. The solution is pruning. Pruning is a technique for reducing the number of items and combinations of items being considered at each step. At each stage the algorithm throws out a certain number of combinations that do not meet some threshold criterion. The most common pruning threshold is called minimum support pruning. Support refers to the number of transactions in the database where the rule holds. Minimum support pruning requires that a rule hold on a minimum number of transactions. For instance if there are one million transactions and the minimum support is 1 percent then only rules supported by 10 000 transactions are of interest. This makes sense because the purpose of generating these rules is to pursue some sort of action such as striking a deal with Mattel the makers of Barbie dolls to make a candy-bar-eating doll and the action must affect enough transactions to be worthwhile. The minimum support constraint has a cascading effect. Consider a rule with four items in it if A B and C then D. Using minimum support pruning this rule has to be true on at .
