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Cancer Screening

There is a growing recognition among medical professionals that cancerscreening 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 gooddecisions

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 detectoccult

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

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

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

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

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

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

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

screening. What is the probability that this person actually hascolorectal


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

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

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

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

percentage of individuals with a disease who arecorrectly 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 personactually has colorectal cancer given a

positive test.

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

routine cancer screening tests for healthypersons are always a good idea.

Your essay will be scored based on your (1)interpretation, (2) calculation, and (3)

communication; see the attached rubric. Butdon’t simply do what the rubric tells you to

do. Be creative: up to 3 additional points are based on

effort, originality, or insight

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