ThisExclusion sectionInclusion is concerned with eventsEvents which are defined in terms of other eventsEvents  \(A_1, A_2, \dots , A_k\) . Of the k events, one, two or more can occur simultaneously. This means there can be overlappingOverlapping events. For example if \(A_1\) and \(A_2\) are two events, then \(A = A_1 \cup A_2\) denotes that either \(A_1\) or \(A_2\) or both can occur. Here \(A = A_1 \cup A_2 \cup \cdots \cup A_k\) and the problem is to find |A|. We can in the beginning find the cardinalityCardinality of A by some direct method. But what is obtained is only an approximate counting. We need to subtract off an overcounting, as the eventsEvents \(A_1, A_2, \dots , A_k\) are overlappingOverlapping. This method will be named as ‘method of InclusionInclusion-ExclusionExclusion’ or ‘Principle of Inclusion-Exclusion’. (PIE)