Selling more isn't making more
So revenue went down when the ads stopped, and profit went up. We sold about £132 less over
those 18 days and saved £207.74 of ad spend to do it: a net gain of roughly £150 in
profit, and it lands on the profit side whether you assume a 35% or a 45% margin.
Both windows, because quoting only the flattering one is how you get caught. The wider 40-day window
sits right at break-even and is a wash: it compares a seasonal peak against a trough, and stock was
drawing down toward a stockout across it, which suppresses the later window for reasons that have
nothing to do with ads. The like-for-like controls for the calendar and is the one I'd stand on.
Neither was a designed experiment, and there was no holdout.
That's the whole case for measuring your own numbers.
A claimed 7.45x and a real 0.63x can't both be true, and only one of them was ours to
check.