Three main metrics have been developed that measure the importance, or accuracy of an association rule: support, confidence, and lift.
Support measures how frequently the items occur together. Imagine having a shopping cart in which there can be a very large number of combinations of items. Some items that occur rarely could be excluded from the analysis. When an item occurs frequently you will have more confidence in the association among the items, since it will be a more popular item. Often your analysis will be centered around items with high support.
Calculating support is simple. You first calculate a proportion by counting the number of times that the items in the rule appear in the basket divided by the total number of occurences in the itemsets: