The boxes with the green ink indicate probabilities that nature has assigned. We might know these from studies that tell us what fraction of the population has a certain illness. The boxes in red ink tell us the undesirable, or costly test mistakes. Clinical or medical trials will give us these probabilities. The boxes with the value of a certain outcome are most likely produced by economic studies that ask people how much they value a year of having that illness versus a year without it, data on the costs of the test and treatments, and the effectiveness of the treatment: combinations of medical and economic data (we will be talking about this information more in class). Finally, the expected value is simply the probability that a person reaches that end point, or end node, times the value of that outcome.

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