Answering Biostatistics Questions on the USMLE Step 1 and Step 2 CK exams

by / Wednesday, 23 June 2010 / Published in Biostatistics

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Success in answering the biostatistics questions comes from not only memorizing the following chart, but actually understanding them.  If you can grasp what is happening you will not have any issues in answering biostats questions in this section of the exams

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True Positive: is the # of people who have the disease with +ve results

False Positive: is the # of people who in fact do not have the disease with a +ve test result

True Negative: is the # of people who do not have a disease who tested –ve

False Negative: is the # of people who have the disease who tested -ve

Sensitivity [A/A+C]

Sensitivity is a tests ability to detect a disease

Specificity [D/B+D]

Specificity is a tests ability to detect health

Positive Predictive Value [A/A+B]

The positive predictive value (PPV) detects the likelihood that the patient has a disease when they test positive for a test

Negative Predictive Value [D/C+D]

The negative predictive value measures how likely a patient is in fact healthy after a test result comes back negative.

Odds Ratio [(a X d) / (b X c)]

Compares the incidence of disease in people exposed X incidence of non-disease in people not exposed, divided by the incidence of people unexposed and incidence of non-disease in those exposed.

OR >1 = the factor being studied is a risk factor for the outcome

OR < 1 = the factor being studied is a protective factor in respect to the outcome

OR = 1 = no significant difference in outcome in either exposed or unexposed group

Relative Risk [a/(a+b) / d/(c+d)]

Compares disease risk in people exposed to a certain factor with disease risk in people who have not been exposed

Attributable Risk [a/(a+b) – d/(c+d)]

The attributable risk is the # of cases attributable to one risk factor

2 Responses to “Answering Biostatistics Questions on the USMLE Step 1 and Step 2 CK exams”

  1. Abhijit Chowdhury says : Reply

    Very helpful article. Thank you.

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