Rework consumes large portions of software engineering budgets. Human factors, and Cognitive Bias in particular, have been shown in other disciplines to be implicated in the kinds of reasoning errors that lead to rework. Research of these phenomena in software engineering lags similar efforts in other disciplines. This study identifies the Performance Norms, standards by which Cognitive Biases are determined to have occurred, in a single but critically important software engineering task: Estimating. Analysis of data from professional practitioners regarding real-life situations indicates that several Performance Norms for Estimating are often ‘in play’, the least important being that assumed in previous, lab-based experiments. Most of these Norms require skills very different from those in which most technical personnel are trained. We conclude that rework reduction efforts will continue to falter until Performance Norms are recognized as key determinants in software engineering practice.