The MECE principle, pronounced "ME-see", 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 is based on ideas going back as far as Aristotle.
The MECE principle is useful 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). 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).
Strategy consultants use MECE problem structuring to break down client problems into logical, clean buckets of analysis that they can then hand out as work streams to consulting staff on the project. Given their extensive use of MECE in breaking down client problems, they also expect to see interview candidates demonstrate strong MECE problem structuring during case interviews. 
The MECE concept has been criticized for not being exhaustive, as it doesn't exclude superfluous/extraneous items.
Also, MECE thinking can be too limiting as mutual exclusiveness is not necessarily desirable. For instance, while it may be desirable to classify the answers to a question in a MECE framework so as to consider all of them exactly once, forcing the answers themselves to be MECE can be unnecessarily limiting.
Another attribute of MECE thinking is that, by definition, it precludes redundancies. However, there are cases where redundancies are desirable or even necessary.
- Proof by cases (case analysis)
- Partition of a set for a mathematical treatment
- Work breakdown structure for application in project management
- Kivestu, Kenton "What is the MECE Principle?" YouTube, Rocketblocks channel
- Cavano, Katharina "3 Steps to a Faster Moving Sales Pipeline"
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- "What is the MECE principle? A deep dive into the problem structuring framework". rocketblocks.me. May 23, 2017.
- van Gelder, Tim (June 4, 2010). "What is MECE, and is it MECE?". timvangelder.com.
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- Chevallier, Arnaud (2016). Strategic Thinking in Complex Problem Solving. Oxford, UK: Oxford University Press. p. 78.