What MECE means
MECE — Mutually Exclusive, Collectively Exhaustive — is the test every consulting interviewer silently applies to your structure. Mutually exclusive means your buckets don't overlap: a cost driver shouldn't also appear under revenue. Collectively exhaustive means, taken together, your buckets cover the whole problem with nothing important left out.
How to make a structure MECE
Split by one dimension at each level. For a profit problem, the first split is Revenue vs. Cost — not Revenue vs. Marketing, which mixes a financial line with a function. Then check two things: does any item belong in two buckets (an exclusivity failure), and is anything missing (an exhaustiveness failure)? This checker flags both.
Worked example
"Why is profit down?" → Profit = Revenue − Cost. Revenue splits into Price × Volume; Volume into existing vs. new customers. Cost splits into Fixed vs. Variable. That tree is MECE. Compare it to "Revenue, Customers, Pricing, Competition" — those overlap (pricing sits inside revenue) and leave costs out entirely.