Round-off error, blind faith, and the powers that be: a caution on numerical error in coefficients for polynomial curves fit to psychophysical data.

Graphing and statistics software often permits users to fit polynomial curves, like a parabola or sigmoid, to scatter plots of psychophysical data points. These programs typically calculate the curve using double- or extended-precision numerical algorithms and display the resulting curve overlaid graphically on the scatter plot, but they may simultaneously display the equation that generates that curve with numerical coefficients that have been rounded off to only a few decimal places. If this equation is used for experimental or clinical applications, the round-off error, especially on coefficients for the higher powers, can produce anomalous findings due to systematic and extreme distortions of the fitted curve, even artifactually reversing the algebraic sign of the true slope of the fitted curve at particular data points. Care must be exercised in setting round-off criteria for coefficients of polynomial terms in curve-fit equations to avoid nonsensical measurement and prediction.