Ratio metrics
A rate can change because the rate itself moved, or because the mix underneath it moved.
Two causes, one number
Conflating a within-segment rate change with a change in the mix of segments is the most common way conversion analysis goes wrong. WHYSE decomposes a ratio metric into both components separately, so "conversion fell" is answered as either "conversion fell within existing segments" or "the mix shifted toward lower-converting segments," not both at once.