Practical Predictive Analytics

Confidence

Confidence is the conditional probability that the event on the right-hand side (rhs or consequence) will occur, given the items on the left-hand has occurred (lhs or antecedent). This can be computed manually by counting the number of occurrences in the transactions.

For example, let's take a closer look at the following rule:

• {tropical fruit} => {bottled water}
• We can see that tropical fruit occurs in three separate transaction itemsets, itemset 2, 3, and 6 and therefore the denominator of the formula is 3
• Of those three itemsets, bottled water occurs in two of them (itemsets 2 and 3 only, but not 6), so the confidence is 2/3 or 67% and also note that the confidence for the reverse itemset {bottled water} => {tropical fruit} ...