Differential Thermal Analysis (DTA) is a technique used to quantify cold tolerance in plants. Plant tissue is cooled, and the ambient temperature and the temperature differential between tissue and ambient are measured. Freezing episodes, called exotherms, can be identified as changepoints, local minima, or selected inflection points in a plot of differential temperature against ambient temperature. The primary exotherm typically manifests itself as a discontinuity and can be identified using techniques similar to those described by Müller (1992) and Loader (1996). Modifications for locally weighted polynomial regression are proposed here to locate additional exotherms through estimation of first and second derivatives. These techniques involve an innovative combination of local and global bandwidth selection. Finally, the estimators are applied to crepe myrtle (Lagerstremia indica) and pecan (Carya illinoensis) DTA data.

Key Words
Bandwidth; Changepoint; Derivative estimation; Local polynomial.

Patrick D. Gerard is Assistant Professor of Experimental Statistics, Experimental Statistics Unit, Box 9653, Mississippi State University, MS 39762-9653. William R. Schucany is Professor of Statistics, Department of Statistical Science, Southern Methodist University, Dallas, TX 75275-0332.