Towards association rules

All of this frequent itemset stuff is fine, but we are ultimately on the hunt for association rules, which are much more exciting. Association rules are formed from frequent itemsets, with a few small twists. We are interested in making a statement about the frequent itemsets like this: people who buy vanilla wafers also buy bananas 60% of the time. In order to do so, we need to learn how to calculate a few additional metrics, starting with two we call support and confidence.

Support

If we are looking for frequent itemsets, then we also need a way to express how often we see these sets occurring in baskets, and whether that number qualifies as frequent. If I see {vanilla wafers, bananas} in 90% of baskets, is that considered frequent? What about 50% of baskets? What about 5%? We call this number the support of the itemset. The support is just the number of times we saw that itemset over all the baskets.

To make support more meaningful, and to begin talking about ...