MECE is a list. It is a list of qualities lists should have. According to MECE, any list should be

Mutually exclusive – the members of the list should “exclude” each other, i.e. be distinct Collectively exhaustive – the members of the list should “exhaust” the relevant field, i.e., contain everything that belongs on the list.

In plain English, MECE says that a list should have

No overlaps No gaps

MECE is widely used by management consultants. In fact it seems to have acquired the status of holy writ. According to Ethan Rasiel in The McKinsey Way:

MECE…is a sine qua non of the problem-solving process at McKinsey. MECE gets pounded into every new associate’s head from the moment of entering the Firm. Every document (including internal memos), every presentation, every email and voice mail produced by a McKinsey-ite is supposed to be MECE. Ask any number of McKinsey alumni what they remember most about the way the Firm solves problems and they will tell you, “MECE, MECE, MECE.

It is therefore interesting to ask whether MECE is adequate, even by its own lights. So we should ask:

Does MECE have any overlaps?

Does MECE have any gaps?

It seems clear that gaps and overlaps are very different, so MECE looks to be ME. But is it CE? We need to ask whether there are any properties that a properly formed list should have over and above ME and CE.

And obviously there are. For example, what’s wrong with the following list?

Bashful Doc Dopey Grumpy Hannibal Lecter Happy Sleepy Sneezy

The list is ME – all items are genuinely distinct. And it is CE – it “exhausts” the relevant field, which is the seven dwarfs. But it is clearly a stupid list. It violates the commonsense principle that lists should not include things that don’t belong on the list.

And it is not too hard to think of other desiderata which are missing from MECE.

So MECE is not merely not MECE. MECE is obviously not MECE. MECE does not live up to its own standards.