Last week I wrote that merging three LoRA specialists "regresses". Then a reader asked the question I had skipped: not merged vs base, but merged vs specialist, paired on the same 20 records.
I ran it. All three intervals include zero (vulnerability +0.018 [−0.005, +0.042], deletion +0.016 [−0.029, +0.061], sensitive_publication +0.005 [−0.010, +0.021]). At n=20 the data can't show the merge lost anything, and can't show it didn't. "Regresses" in the title is untested.
Two things I checked next. A cat merge with weights [1,1,1] gives exactly the sum of the three specialist deltas (rank 48, error 4e-8), so it tests "no cross terms" and linear 1/3 tests something else. And I went looking for where the "gold" probabilities came from, because the whole test measures agreement with them, and what I found changes how to read every number above. Details, script and raw files are in the repo.
The fix is a fresh test: ~80 new chains, the old 20 kept outside, run after Oct 6.