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.