Modelling variation in the METR Uplift Study

Disclaimer: Not associated with METR. Summary I reanalysed the METR uplift study under a meta-analytic model to estimate the range of plausible effects that we may see in repeated trials. If the average population effect varied as much as medical and economic Randomized Control Trials (RCTs), we would see a 95% credible interval of results from -55% to +193% even without study-specific effects. The mean population effect was between 10-20% with heavy tails. The probability of a slowdown is between 66% and 79%. The study was therefore compatible with both large speedups and slowdowns, even with no further AI progress. We should be careful about drawing inferences on the sign or magnitude of AI-assistance when study variation is high, but this may explain the radically different effectiveness heard from self-reports. It would be good to find more ways of squeezing more information out of older data, since it is unlikely for us to gain more before the point of no return (if that isn't already passed). AI speedups and slowdowns The METR RCT remains the highest quality evidence that we have for developer productivity improvements using AI tools. The original METR uplift RCT was published in July 2025. It measured developer completion times from a selected set of issues from open source software projects. The headline result was developers were slower by 19% when using AI tools. This surprised many people, including expert forecasts, METR, and the developers themselves. Given the widespread use of AI tools in software now, it is unlikely that these measurements will ever be repeated. Now, post Opus 4.8, Fable, Sol etc., there is a growing feeling that developers are sped up by at least 50% if not more. How can we know when we have definitively shifted in ability between models? How to meta-analyse a single study When we carry out an experiment[1], the end result is a measured effect from one variable to another. In the METR study, this effect is the logged ratio of the AI-assisted vs AI-disabled time-to-complete across all the projects. Let's call this mjx-container[jax="CHTML"] { line-height: 0; } mjx-container [space="1"] { margin-left: .111em; } mjx-container [space="2"] { margin-left: .167em; } mjx-container [space="3"] { margin-left: .222em; } mjx-container [space="4"] { margin-left: .278em; } mjx-container [space="5"] { margin-left: .333em; } mjx-container [rspace="1"] { margin-right: .111em; } mjx-container [rspace="2"] { margin-right: .167em; } mjx-container [rspace="3"] { margin-right: .222em; } mjx-container [rspace="4"] { margin-right: .278em; } mjx-container [rspace="5"] { margin-right: .333em; } mjx-container [size="s"] { font-size: 70.7%; } mjx-container [size="ss"] { font-size: 50%; } mjx-container [size="Tn"] { font-size: 60%; } mjx-container [size="sm"] { font-size: 85%; } mjx-container [size="lg"] { font-size: 120%; } mjx-container [size="Lg"] { font-size: 144%; } mjx-container [size="LG"] { font-size: 173%; } mjx-container [size="hg"] { font-size: 207%; } mjx-container [size="HG"] { font-size: 249%; } mjx-container [width="full"] { width: 100%; } mjx-box { display: inline-block; } mjx-block { display: block; } mjx-itable { display: inline-table; } mjx-row { display: table-row; } mjx-row > * { display: table-cell; } mjx-mtext { display: inline-block; } mjx-mstyle { display: inline-block; } mjx-merror { display: inline-block; color: red; background-color: yellow; } mjx-mphantom { visibility: hidden; } _::-webkit-full-page-media, _:future, :root mjx-container { will-change: opacity; } mjx-math { display: inline-block; text-align: left; line-height: 0; text-indent: 0; font-style: normal; font-weight: normal; font-size: 100%; font-size-adjust: none; letter-spacing: normal; border-collapse: collapse; word-wrap: normal; word-spacing: normal; white-space: nowrap; direction: ltr; padding: 1px 0; } mjx-container[jax="CHTML"][display="true"] { display: block; text-align: center; margin: 1em 0; } mjx-container[jax="CHTML"][display="true"][width="full"] { display: flex; } mjx-container[jax="CHTML"][display="true"] mjx-math { padding: 0; } mjx-container[jax="CHTML"][justify="left"] { text-align: left; } mjx-container[jax="CHTML"][justify="right"] { text-align: right; } mjx-mi { display: inline-block; text-align: left; } mjx-c { display: inline-block; } mjx-utext { display: inline-block; padding: .75em 0 .2em 0; } mjx-msub { display: inline-block; text-align: left; } mjx-mo { display: inline-block; text-align: left; } mjx-stretchy-h { display: inline-table; width: 100%; } mjx-stretchy-h > * { display: table-cell; width: 0; } mjx-stretchy-h > * > mjx-c { display: inline-block; transform: scalex(1.0000001); } mjx-stretchy-h > * > mjx-c::before { display: inline-block; width: initial; } mjx-stretchy-h > mjx-ext { /* I