From the other side of the consultation, briefly. The useful move here is to separate what is established from what is widely repeated. Those two sets overlap less than the confident tone of most write-ups suggests, and the second set is where nearly all the disagreement on this board comes from.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

Dr.PulmRoch said:

The useful move here is to separate what is established from what is widely repeated.

Second this.

Dr.PulmRoch said:

The useful move here is to separate what is established from what is widely repeated.

Adding the part of the answer the thread has not reached. The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either. Those three things can all be true at once, and most arguments here are two people holding different parts of that.

One concrete data point for the thread. One habit that pays for itself: post the method alongside the number. A figure without its method cannot be checked, and an unchecked figure is how this community accumulates folklore.

Moderator note: leaving this open. It is being argued well and the disagreement is the useful part. Tagging this one for the weekly digest.