www-ai.cs.tu-dortmund.de/LEHRE/SEMINARE/SS09/AKTARBEITENDESDM/LITERATUR/item_sets_that_compress.pdf
sets:
• CS1 = {{I1, I2}, {I1}, {I2}, {I3}}
• CS2 = {{I1, I2, I3}, {I1, I2}, {I1}, {I2}, {I3}}
• CS3 = {{I1, I2, I3}, {I1}, {I2}, {I3}}
Assume that supp({I1, I2, I3}) = supp({I1, I2}) + 1. It is very well [...] formalised in lemma 3.1.
Lemma 3.1. Let I be a set of items and db a database of transactions over I. Moreover, let J and K be two
Standard(I, db) 1 foreach I ! I 2 freq(I) := support of I in db 3 Res := I ordered [...] it is not the whole story. Because, what happens if we now add an item set J3 before J2
such that:
• J1 ) J3 = * and
• J2 ) J3 '= *.
The frequency of J2 will go down, while the frequency of J1 will go up …