Estimating Physiological Thresholds with Continuous Two-Phase Regression

Abrupt changes in the relationship between physiological responses and environmental parameters yield data that frequently cannot be described with a single regression equation. Many approaches used to deal with this problem result in incomplete description of the data and imprecise approximations of the physiological threshold(s) at which the relationship changes. We describe a technique for determining the best continuous two-phase, straight-line regression model and for statistically estimating the point at which the relationship between the independent and dependent variables changes (i.e., threshold point).