A spinner should be judged on the ball before

Wickets get credited to the delivery that took them, which is usually the least interesting one in the over.

A batter drives loosely and edges to slip. The commentary praises the ball, the replay shows the ball, the wicket is filed against the ball. Almost nobody goes back two deliveries to the one that was six inches fuller and pushed him half a pace forward, which is the only reason the drive happened at all.

Spin bowling is the format where this matters most and where we are laziest about it. Seam bowling has enough randomness in it that a good ball can arrive from nowhere. Spin does not really work like that. A wicket is usually the fourth or fifth step of something that began earlier in the over, and the earlier steps look, on their own, entirely unremarkable.

This creates a genuine measurement problem for anyone selecting a side. If you judge on wickets you are judging on the last event in a chain. Two bowlers can build identical pressure and one of them gets the reward because a batter happened to mishit. Over a season that evens out slightly. Over a series it does not even out at all, and selections get made on series.

The number that would actually help is boring and nobody publishes it. How often does this bowler get a batter to play at a ball he did not have to play at. That is the mechanism. Everything else, the turn, the dip, the variations, is in service of producing that one thing, and it is countable if anybody could be bothered.

I would also say this is why young spinners get coached badly. Told to take wickets, they bowl the wicket-taking ball too early and too often, before anything has been set up, and it gets swept for four because it arrived without context. The delivery was not bad. It was premature, which is a different fault and requires a different correction.

Watch an over backwards sometime. Start at the wicket and work back to where the batter was actually comfortable. The distance between those two points is the bowler, and none of it is on his figures.