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# roc_module - ROC Module 1 Indexes How do you determine if...

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October 28, 2002 1 ROC Module

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October 28, 2002 2 Indexes How do you determine if you are healthy or sick? One way is by using an index. An index is a meausre that is used to diagnosis for diseases or “abnormalities.” For instance, the police use an index of blood alchohol concentration to determine if someone has been drinking and driving. A blood alchool concentration above .08 means a person has .08 grams of alcohol per 100 ml of blood. In most states this is considered legally drunk. Indexes are used often to determine if we have a disease.
October 28, 2002 3 Creating a Test Suppose that you have designed a test for the hyopthetical disease “x.” According to the “cut off” a person who has more than .47 mg/ml of “y” in their blood is considered sick, while a person who has less is con- sidered helalthy. 0. 1. 2 . 3. 4 . 5. 6. 7. 8 .9 Index Healthy Sick 1.00 Cut-Off Line Healthy Sick Everyone to the right of the line is considered healthy. Everyone to the left of the cut-off is considered sick. This index for disease “x.” In order to diagnose “x” we measure the amont of “y” in the blood. (g/ml of “y”) There is a cut-off line at .47g/ml.

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October 28, 2002 4 How Accurate is Our Test? Indexes are calculated based on the population as a whole. As a result there is bound to be some errors. Sometimes sick people will be diagnosed as healthy, and healthy people will be diagnosed as sick. The trick is to set the cut-off line on the index so that there are as few errors as possible. In the example below, we can see how our test results compare to the “true” population.” 0. 1. 2 . 3. 4 . 5. 6. 7. 8 .9 Index 1.00 Cut-Off Line Healthy Sick Clinical Test True Population
October 28, 2002 5 If you were going to the doctor and had a test for a disease, which one would you most fear? Sick Sick In “Truth” or Reality According to test 1. Sick Healthy 2. Healthy Healthy 5. Sick Sick 6. True Positive This person is sick in reality. The test says that the person is sick. False Positive This person is healthy in reality.

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roc_module - ROC Module 1 Indexes How do you determine if...

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