Q2 What is support and confidence ?

Answer

**Support and Confidence**

Several objective measures of pattern interestingness exist. These are based on the structure of discovered patterns and the statistics underlying them. An objective measure for association rules of the form X ⇒ Y is rule support, representing the percentage of transactions from a transaction database that the given rule satisfies. This is taken to be the probability P(X ∪Y), where X ∪Y indicates that a transaction contains both X and Y, that is, the union of itemsets X and Y. Another objective measure for association rules is confidence, which assesses the degree of certainty of the detected association. This is taken to be the conditional probability P(Y|X), that is, the probability that a transaction containing X also contains Y. More formally, support and confidence are defined as

support(X ⇒ Y) = P(X∪Y),

confidence(X ⇒ Y) = P(Y|X).

In general, each interestingness measure is associated with a threshold, which may be controlled by the user. For example, rules that do not satisfy a confidence threshold of, say, 50% can be considered uninteresting. Rules below the threshold likely reflect noise, exceptions, or minority cases and are probably of less value.

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