人工智能教学资料 8ai Association Analysis part2.pdfVIP

1Compact representation of frequent itemsets ? The number of frequent itemsets produced from a transaction data set can be very large. ? It is useful to identify a small representative set of frequent itemsets from which all other frequent itemsets can be derived. ? Two representations are ? Maximal frequent itemsets ? Closed frequent itemsets 2Maximal frequent itemsets ? A maximal frequent itemset is defined as a frequent itemset for which none of its immediate supersets are frequent. ? We consider the itemset lattice shown in the following figure. ? The itemsets in the lattice are divided into two groups ? Those that are frequent ? Those that are infrequent 3Maximal frequent itemsets 4Maximal frequent itemsets ? A frequent itemset border is also illustrated in the figure. ? Every itemset located above the border is frequent. ? On the other hand, those located below the border are infrequent. 5Maximal frequent itemsets ? {a,d}, {a,c,e} and {b,c,d,e} are considered to be maximal frequent itemsets. ? This is because their immediate supersets are infrequent. ? In contrast, {a,c} is non-maximal because one of its immediate supersets, {a,c,e}, is frequent. 6Maximal frequent itemsets ? Maximal frequent itemsets effectively provide a compact representation of frequent itemsets. ? They form the smallest set of itemsets from which all frequent itemsets can be derived. 7Maximal frequent itemsets ? We can divide the frequent itemsets in the previous figure into two groups. ? The first group consists of frequent itemsets that ? Begin with item a and ? Followed by items c, d or e. ? This group includes itemsets such as {a}, {a,c}, {a,d}, {a,e} and {a,c,e}. ? The second group consists of frequent itemsets that ? Begin with b, c, d, or e. ? This group includes itemsets such as {b}, {b,c}, {c,d}, {b,c,d,e}, etc. 8Maximal frequent itemsets ? Frequent itemsets that belong to the first group are subsets of eit