8 Chapter 1 IntroduCtIon to data MInIng
The formal definition of the problem of data mining for association
rules can be stated as follows: Let Z be a set of items that can be purchased,
and let D be a set of transactions for a given period. Let T ∈ D and T ⊆ Z.
A unique identifier called a TID is assigned to each transaction. Mining for
association rules refers to the process of extracting rules of the form X ⇒ Y
from databases containing raw data, where X, Y ⊆ Z, and X ∩ Y = ∅. There
are two factors that affect the significance of the association rules extracted:
support and confidence. We say that rule X ⇒ Y has support s in transaction
set D if s% of the transactions in D contain X ∪ Y. On the other hand, we
say that rule X