GitHub Copilot and Developer Productivity: An Observational Dose-Response Analysis
Does GitHub Copilot (GHCP) make engineers more productive, or do the engineers who use it more differ from those who use it less? And even within a single engineer, are GHCP-heavy weeks just busy weeks in which more of everything gets done? We study these questions using 43 weeks of data from 16,223 software engineers across Microsoft's Cloud+AI organization. Engineer fixed effects address the first concern by comparing each engineer against themselves rather than against other engineers, eliminating time-invariant differences in skill, role, and team. Active coding time and browser time then enter a Poisson Pseudo-Maximum Likelihood model with two-way fixed effects to address the harder, within-engineer confound: that GHCP-heavy weeks coincide with high-effort weeks. This defines our estimand as an efficiency effect: more pull requests completed at equivalent levels of coding time. Engineers are estimated to complete 40.5% more PRs in their highest GHCP usage weeks relative to their zero-usage weeks, holding measured development effort constant. The gradient is monotonic with diminishing returns at high intensity. Seven robustness and falsification tests target the remaining plausible alternative explanations (non-coding AI engagement, team-level shocks, within-week task reallocation, cross-week contamination, PR slicing into smaller units, shifts toward easier task types, and sensitivity to how the treatment is operationalized). Under an explicitly stated conditional-independence assumption, the within-engineer design estimates a tool-specific efficiency effect that is consistent with all seven robustness tests.