Practical Big Data Analytics

Confidence

Confidence refers to how often the right-hand side of the apriori expression is valid when the left-hand side is valid. For instance, given an expression:

{Milk} à {Bread}

We would like to know how often Bread was purchased when Milk was also purchased.

In this case:

• Transaction 1: Milk and Bread are both present
• Transaction 2: Milk is present, but not Bread
• Transactions 3 and 4: Milk is not present

Hence, based on the what we saw, there were 2 transactions where Milk was present and of them, Bread was present in 1 transaction. Hence, the confidence for the rule {Milk} à {Bread} would be ½ = 50%

Taking another expression:

{Bread} à {Butter}

We would like to know, when Bread was purchased, how often was Butter also purchased?: ...