On comparing regression lines with unequal slopes.

For the situation in which the slopes of two regression lines may differ, procedures are discussed for the comparison of the lines at a single X, simultaneous comparison at several values of X, and determination of the range of X for which the difference between the lines is large enough to be statistically significant. These useful procedures have previously been discussed but do not seem to be well known. Their usefulness in the analysis of physiological data is demonstrated with data arising from a study of pulmonary response to hypoxia.