Again, my goal was to make a decision, so I had to assign decisions based on what the data could show me.
It seems to me like you came up with a sensible metric for determining whether posts should be made longer or shorter, conditional on the post length changing, but that it would be better to determine also whether or not post length should change. That’s sort of what the 25% cutoff was pointing at, but note that it doesn’t distinguish between the world where it’s split 60-5-35 (for longer-same-shorter) and the world where it’s split 25-75-0. The first world looks like it needs you to split out your readership and figure out what the subgroups are, and the second world looks like you should moderately increase post length.
(Of course, to actually get the right decision you also need the cost estimate for being too long vs. too short; one might assume that you should tinker with the length until the two unhappy groups are equally sized, but this rests on an assumption that is often wrong.)
It seems to me like you came up with a sensible metric for determining whether posts should be made longer or shorter, conditional on the post length changing, but that it would be better to determine also whether or not post length should change. That’s sort of what the 25% cutoff was pointing at, but note that it doesn’t distinguish between the world where it’s split 60-5-35 (for longer-same-shorter) and the world where it’s split 25-75-0. The first world looks like it needs you to split out your readership and figure out what the subgroups are, and the second world looks like you should moderately increase post length.
(Of course, to actually get the right decision you also need the cost estimate for being too long vs. too short; one might assume that you should tinker with the length until the two unhappy groups are equally sized, but this rests on an assumption that is often wrong.)