Abstract
The ITSE method (Gottman, 1981; Rushe & Gottman, 1993; Williams & Gottman, 1982, 1999) for analyzing the 2-phase interrupted time-series design was evaluated. I point out that the method yields intervention effect estimates that are inconsistent with the logic of the design and that these estimates are likely to be quite misleading. Furthermore, the omnibus F test and the subsequent t test on level change are inconsistent in purpose; the omnibus F statistic is shown not to be relevant to the stated hypothesis of equality of intercepts defined for the method. The omnibus F can be exceedingly large when subhypotheses regarding equality of slopes and intercepts are true; the converse can also occur. Related problems, such as inconsistencies in published simulation evaluations and problems with software, are explained. It is recommended that the ITSE method not be used regardless of sample size.
