Chapter 9: Measuring utilities in cost–utility analysis
The number of QALYs associated with any health state can be expressed as the product of the number of years T (for time) in that state and the average quality of a year in that state, which will be denoted as _, that is, QALYs 5 _ 3 T. This formulation adopts the constant proportionally (CP) assumption for _ whereby any utility value is just multiplied by time (number of years). We will (unlike some of the literature) use the term quality of life, or QoL, interchangeably with utility and value. This chapter is devoted to explaining, comparing and contrasting the three main methods for deriving estimates for _. By a method we mean the type of question asked of a person to extract his or her preferences and the way this gets recorded as a number (usually) between 0 and 1. We will not be referring to alternative statistical estimation techniques as different methods. The three methods covered are the category rating scale (RS), the standard gamble (SG) and the time trade-off model (TTO). We make comparisons in terms of both theory and practice. Before explaining each method in detail, and then presenting the applications, we cover some background issues.
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