In August 1976, tuberculosis was diagnosed in a high school student (index case) in Corinth, Mississippi. Subsequently, laboratory studies revealed that the student's disease was caused by drug-resistant tubercule bacilli. An epidemiologic investigation was conducted at the high school. Table E1.5 gives the rate of positive tuberculin reactions, determined for various groups of students according to degree of exposure to the index case.
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TABLE E1.5
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Exposure Level
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Number Tested
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Number Positive
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High
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129
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63
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Low
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325
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36
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(a) Compute and compare the proportions of positive cases for the two exposure levels. What would be your conclusion?
(b) Calculate the odds ratio associated with high exposure. Does this result support your conclusion in part (a)?