ESDU 92021 uses the analysis of a set of measurements of approach speed and associated landing ground roll distance to illustrate the use of linear, two variable regression. Regression is used twice: firstly, on an empirical basis, a linear relationship is assumed between the two variables, and, secondly, taking account of the physics of the manoeuvre, a linear relationship is assumed between the square of the approach speed and the ground roll. In both cases, the significance of the derived fit is verified using the F variance ratio test and the correlation coefficient. Both fits are found to be excellent, although the latter is clearly the better physical model. An introduction to statistical techniques for use in the analysis of aircraft performance is given in ESDU 91017.