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Tools for Decision Making Sections
Author Bio
Introduction
Part I
Expected Value Decision Making
Case Study 2: Patient History
Using a Decision Tree
Assigning Utility
Currently selected section: Expected Value and QALYs
Sensitivity Analysis
Conclusions: Case Study 2
Part III

References


Chapter 14: Tools for Decision Making: Expected Values and QALYs
        

Next, we illustrate how to calculate expected value and QALYs for one possible outcome.

To calculate these figures for the "Do Transplantation" option (see diagram below), use the assumptions and instructions that follow.

Figure 2.5.1: Calculating Expected Value and QALYs
Decision tree for Do transplantation, details in text.

Assumptions ("Do Transplantation"):

  • UTrans = 0.9
  • UloseFoot = 0.8
  • Assume that rejection occurs in the first few months after surgery and neglect the disutility of a few months of anti-rejection treatment for the "reject" branch of the second chance node.


Step 1: Calculate the expected value of the right hand node. The technical term for this process is "averaging out and folding back."

  • "Averaging out" means that you multiply the QALYs for one outcome by the probability of that outcome and sum over all the branches at the chance node.

  • "Folding back" means that you move from right to left, from the tips of the tree to the root of the tree, multiplying the probability of an event by the QALYs of the event.

For example, a person who survives the transplantation operation can expect to live 8.40 QALYs on average (see below).

Figure 2.5.1a: Calculating Expected Value and QALYs
Decision tree for Do transplantation. Calculating expected value and QALYs where step 1 ([0.4 x 9] + [0.6 x 8] = 8.40) occurs at the right hand node (success or reject), details in text.


Step 2: Calculate the QALYs at the left hand node, multiplying the QALYs at the right hand node by the probability of surviving the transplantation operation. The QALYs for the "Do Transplantation" option are 8.23 life years (see below).

Figure 2.5.1b: Calculating Expected Value and QALYs
Decision tree for Do transplantation. Calculating expected value and QALYs where step 2: ([0.98 x 8.40] + [0.02 x 0] = 8.23) occurs at the left hand node (transplantation operation achieved or operative death), details in text.


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