PartnerRoundMECE, properly understood
Mutually exclusive, collectively exhaustive, and why most candidates only get the second half right.
MECE means your buckets do not overlap (mutually exclusive) and together cover the whole problem (collectively exhaustive). Candidates usually manage exhaustive and fail exclusive, producing structures where the same driver appears in three branches and the analysis goes in circles.
Diagnosing a declining-profit case
Break down why profit is falling.
Strong
Profit = Revenue - Cost. Revenue = Volume × Price. Cost = Fixed + Variable. Every driver has exactly one home, and the arithmetic ties out, so any finding can be sized.
Weak
'Customers, competitors, costs, and marketing.' Marketing spend is a cost; competitors act through volume and price. The same effect can be filed in three places, so nothing can be quantified cleanly.
For each branch ask: 'if I found the answer here, could it also be true in another branch?' If yes, your cut is wrong. Re-cut along something arithmetic (revenue vs cost, new vs existing customers, own vs competitor action) rather than something thematic.
One nuance worth internalising: perfect MECE is a tool, not a religion. A structure that is 95% exhaustive but genuinely tailored to this client beats a textbook-clean framework that could have been drawn before the prompt was read. Interviewers reward the tailoring.