In Reply: We are grateful to learn of Dr. Larkin's response to our article. In reading his letter, we have no doubt as to the veracity of his account of the origins of the quadrant model. At the time we wrote and submitted the manuscript to Psychiatric Services, we had only material published in the medical and scientific literature to which to refer. We drew in particular from a 2002 report to Congress by the Substance Abuse and Mental Health Services Administration, an article published by Richard Ries, and a chapter written by Richard Rosenthal and Laurence Westreich, each of which described and outlined the model organized by four quadrant boxes of psychiatric and substance abuse severity. Subsequent conversations with Dr. Rosenthal have confirmed the contribution of Dr. Larkin's seminal idea to the original model, which over the years has undergone considerable refinement and elaboration.

We believe this model has widespread conceptual and heuristic value, and as we reported in our article, if it is connected with system-level data, it may also have pragmatic reliability and validity. We commend Dr. Larkin for his significant contribution to the creation of the model more than 20 years ago, and we are pleased to be a part of its continuing application in policy, services, and research with persons who have co-occurring psychiatric and substance use disorders.