Establishing equivalence between scaled measures of quality of life

In this paper, methodologies which have been used in the pharmaceutical industry to demonstrate the equivalence of drug preparations, are applied to the measurement of quality of life (QOL). This approach is feasible when the generated data are measured on the same scale. Data from the quality of life instruments are transformed into interval scales by means of an appropriate scaling procedure. It is demonstrated that equivalence of QOL instruments is linked by a linear relationship between the QOL instruments Functional Assessment of Cancer Therapy (FACT) and the Functional Living Index-Cancer (FLIC). The linear relationship is derived using orthogonal least squares regression which takes into account that both measures are subject to error.