What is food-cost variance and why is it the control number?

Variance is the gap between theoretical food cost (what the recipes say the sales should have consumed) and actual (what the kitchen really used). Read outlet by outlet, it is the single most honest control number in a group: it catches portioning drift, waste and shrinkage in days, and it makes performance comparable across branches in a way raw percentages never are.

From Multi-Outlet Restaurant Groups: Why the Numbers Get Lost — and the Disciplines That Fix It — the full reading, with the method behind this answer.