how to identify base rate

timer Asked: Nov 29th, 2016

Question description

There is a growing recognition among medical professionals that cancer screening is a

double-edged sword. While some individuals may benefit from early detection, others

may be falsely diagnosed and treated for cancer unnecessarily. To make good decisions

about screening, the public needs to know its potential benefits and harms.

The following two paragraphs are taken from a study to test physicians’ ability to

interpret clinical laboratory results.

To diagnose colorectal cancer, the hemoccult test is conducted to detect occult

blood in the stool. For symptom-free people over 50 years old who participate in

screening using the hemoccult test, the following information is available.

The probability that one of these people has colorectal cancer is 0.3 percent. If a

person has colorectal cancer, the probability is 50 percent that he will have a

positive hemoccult test. If a person does not have colorectal cancer, the

probability is 3 percent that he will still have a positive hemoccult test. Imagine a

person (over age 50, no symptoms) who has a positive hemoccult test in your

screening. What is the probability that this person actually has colorectal


Your assignment is to use this hemoccult test scenario as an example to communicate the

nature of a medical test to a lay person. (Think of a family member or a friend who is not

familiar with statistics.) You need to incorporate three elements in your essay:

1. Identify the base rate (or prevalence in medical literature), sensitivity (the

percentage of individuals with a disease who are correctly classified as having the

disease), and false positive rate in this case. Provide a brief and clear description

of these terms using your own words.

2. Calculate the probability that this person actually has colorectal cancer given a

positive test.

3. Based on your analysis, communicate your result to someone who thinks that

routine cancer screening tests for healthy persons are always a good idea.

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