Why Q4 Always Blows Engineering Budget: Per-month K Seasonality
A 60-person engineering org we instrumented through 14 months of finance data ran an average overhead coefficient of K = 0.41. That number is useless. The actual monthly series is Jan 0.46, Feb 0.40, Mar 0.39, Apr 0.40, May 0.41, Jun 0.43, Jul 0.48, Aug 0.49, Sep 0.42, Oct 0.40, Nov 0.43, Dec 0.52. The flat-K finance model predicted December overhead of $185K. Reality came in at $235K. The 27% miss is not a forecasting bug. It is the entire story of every Q4 budget surprise the CFO has ever asked engineering to explain.
DORA's 2024 State of DevOps report flagged the same shape from a different angle: deployment frequency in Q4 drops 12–18% across the surveyed cohort, while incident volume rises. Stack Overflow's 2024 Developer Survey reports developers take an average of 17 vacation days per year, with concentration in late December and August. Harvard Business Review's Why Most Product Launches Fail notes Q4 launch density runs 30–40% above other quarters. Three different datasets, one consequence: engineering capacity in December is structurally different from June. Treating it as the same in your finance model is the mistake.
