For this discussion board, you will be presented with information about a new screening test. For your initial
post, you will perform a series of calculations based on that information and then use those calculations to form
an opinion about the value of the screening test.
Scenario: A new hypothetical instantaneous screening test for marijuana has been developed to identify people
driving under the influence. 100 drivers are stopped by police and screened. 15 people were actually under the
influence of marijuana, while the remainder were not. Of the 15 under the influence, only 10 tested positive. Of
those who were not under the influence, 40 tested positive.
Instructions for initial post: For your initial post, please complete the following questions. Be sure to set up your
two-by-two table. In your post, you must correctly explicitly state the context of every numeric answer by
including a sentence of interpretation (for example: “98% of patients who truly have breast cancer will have a
positive mammography screen”).
What is the prevalence of driving under the influence of marijuana?
Calculate sensitivity and interpret.
Calculate specificity and interpret.
Calculate positive predictive value and interpret.
If more people drove under the influence of marijuana, how would the positive predictive value change?
Calculate negative predictive value.
Calculate the accuracy of the screening tool.
In addition to posting your answers to the above questions, use those answers to write an opinion about:
whether or not this is a reasonable screening tool for police to use to enforce laws regarding driving under the
influence and why?
how this screening tool could be improved?
Sample Solution