MECE

Mutually exclusive and collectively exhaustive: a way of breaking a problem into parts that do not overlap and leave nothing out.

Splitting a decline in profit into volume, price and cost is one such breakdown; splitting it into pricing, competition and the economy is not, because those overlap and none of them is measured.

The discipline is worth learning because it converts an argument into arithmetic. If the parts sum to the whole, the analysis can be checked, and the biggest part tells you where to look next.

Sixty, twenty-five and fifteen. If it does not add to a hundred, the split is wrong.