Rothenberg et al. (2004) investigated the effectiveness of using the Hologic Sah
ID: 3023583 • Letter: R
Question
Rothenberg et al. (2004) investigated the effectiveness of using the Hologic Sahara Sonometer, a portable device that measures bone mineral density (BMD) in the ankle in order to predict small fractures. After testing with this device, patients were given an x-ray to confirm the presence or absence of ;a fracture.
BMD Test result
X-ray confirmation result
Total
Present (D+)
Absent (D-)
Positive (T+)
314
570
884
Negative (T-)
79
336
415
Total
393
906
1299
(a) What is the specificity of this test?
(b) What is the sensitivity of this test?
(c) What is the predictive value of a positive test?
(d) What is the predictive value of a negative test?
(e) A drug is being tested for use in the treatment of poison ivy. Of 190 people tested, 150 gained some relief from the drug. What is the probability that the drug will be effective on the next patient who uses it?
BMD Test result
X-ray confirmation result
Total
Present (D+)
Absent (D-)
Positive (T+)
314
570
884
Negative (T-)
79
336
415
Total
393
906
1299
Explanation / Answer
a. Specificity : [True negative/ (false positive + true negative)] x 100 = (336/906) x100 = 37.08%
b. Sensitivity: [True positive/ (false negative + true positive)] x 100 = (314/393) x 100 = 79.898%
c. predictive value of a positive test = true positive/(true positive + false positive) = (314/884) x 100 = 35.52%
d. predictive value of a negative test = true negatives/(true negatives +false negatives) = (79/415) x 100 =10.04%
e. 150/190 = 78.95%
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