What does MECE mean in Mathematics?

MECE

The MECE principle, (mutually exclusive and collectively exhaustive) pronounced by many as "ME-see", and pronounced by the author as "Meese" like Greece or niece, is a grouping principle for separating a set of items into subsets that are mutually exclusive (ME) and collectively exhaustive (CE). It was developed in the late 1960s by Barbara Minto at McKinsey & Company and underlies her Minto Pyramid Principle, and while she takes credit for MECE, according to her interview with McKinsey, she says the idea for MECE goes back as far as to Aristotle.The MECE principle has been used in the business mapping process wherein the optimum arrangement of information is exhaustive and does not double count at any level of the hierarchy. Examples of MECE arrangements include categorizing people by year of birth (assuming all years are known), apartments by their building number, letters by postmark, and dice rolls. A non-MECE example would be categorization by nationality, because nationalities are neither mutually exclusive (some people have dual nationality) nor collectively exhaustive (some people have none).

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