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1 CPS 196.03: Information Management and Mining Association Rules and Frequent Itemsets

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2 Improvements to A-Priori Park-Chen-Yu Algorithm Multistage Algorithm Savasere, Omiecinski, and Navathe Algorithm

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3 PCY Algorithm uHash-based improvement to A-Priori. uDuring Pass 1 of A-priori, most memory is idle. uUse that memory to keep counts of buckets into which pairs of items are hashed. wJust the count, not the pairs themselves. uGives extra condition that candidate pairs must satisfy on Pass 2.

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4 Picture of PCY Hash table Item counts Bitmap Pass 1Pass 2 Frequent items Counts of candidate pairs

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5 PCY Algorithm --- Before Pass 1 uOrganize main memory: wSpace to count each item. One (typically) 4-byte integer per item. wUse the rest of the space for as many integers, representing buckets, as we can.

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6 PCY Algorithm --- Pass 1 FOR (each basket) { FOR (each item) add 1 to item’s count; FOR (each pair of items) { hash the pair to a bucket; add 1 to the count for that bucket }

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7 PCY Algorithm --- Between Passes uReplace the buckets by a bit-vector: 1 means the bucket count ≥ the support s (frequent bucket); 0 means it did not. uIntegers are replaced by bits, so the bit vector requires little second-pass space. uAlso, decide which items are frequent and list them for the second pass.

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8 PCY Algorithm --- Pass 2 uCount all pairs {i,j } that meet the conditions: 1.Both i and j are frequent items. 2.The pair {i,j }, hashes to a bucket number whose bit in the bit vector is 1. uNotice all these conditions are necessary for the pair to have a chance of being frequent.

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9 Memory Details uHash table requires buckets of 2-4 bytes. wNumber of buckets thus almost 1/4-1/2 of the number of bytes of main memory. uOn second pass, a table of (item, item, count) triples is essential. wThus, we need to eliminate 2/3 of the candidate pairs to beat a-priori.

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10 Multistage Algorithm uKey idea: After Pass 1 of PCY, rehash only those pairs that qualify for Pass 2 of PCY. uOn middle pass, fewer pairs contribute to buckets, so fewer false drops --- frequent buckets with no frequent pair.

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11 Multistage Picture First hash table Second hash table Item counts Bitmap 1 Bitmap 2 Freq. items Counts of Candidate pairs

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12 Multistage --- Pass 3 uCount only those pairs {i,j } that satisfy: 1.Both i and j are frequent items. 2.Using the first hash function, the pair hashes to a bucket whose bit in the first bit-vector is 1. 3.Using the second hash function, the pair hashes to a bucket whose bit in the second bit-vector is 1.

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13 Important Points 1.The two hash functions have to be independent. 2.We need to check both hashes on the third pass. wIf not, the pair could pass tests (1) and (3), yet it was never hashed on the second pass because it was in a low- count bucket on the first pass.

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14 SON Algorithm uSON (Savasere, Omiecinski, and Navathe) uRepeatedly read small subsets of the baskets into main memory wRun Apriori on each subset with a suitably scaled support value uAn itemset becomes a candidate if it is found to be frequent in any one or more subsets of the baskets.

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15 Idea uTake a main-memory-sized chunk of the market baskets. uRun a-priori or one of its improvements (for sets of all sizes, not just pairs) in main memory, so you don’t pay for disk I/O each time you increase the size of itemsets. wBe sure you leave enough space for counts.

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16 The Picture Chunk of baskets Space for counts

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17 SON Algorithm uOn a second pass, count all the candidate itemsets and determine which are frequent in the entire set. uKey “monotonicity” idea: an itemset cannot be frequent in the entire set of baskets unless it is frequent in at least one subset.

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