Ottenbacher (1986) showed the usefulness of single-subject design (SSD) in occupational therapy. However, SSD methodology is not regarded by the wider research community as providing statistically reliable and valid evidence of effectiveness of treatment partly because of its observational nature. Although statistical estimations can also be made from least squares regression or by a trend line, a new methodology has great potential to influence research in occupational therapy. The new model enables the use of initial client data from the beginning of treatment (for single subjects or small groups) to determine a point in the linear regression at which predictions can be made for the number of treatments needed for stability or improvement. This model is invaluable for third-party payment as well as for client motivation. The purpose of this article is to present this new methodology, the semiparametric ratio estimator (SPRE), illustrated by case application to treatment of obesity.