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#1
04-13-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 255 Threads: 26 Thanked 85 Times in 71 Posts Reputation: 95

An imaginary study is conducted to assess the performance of the anti-endomysial antibody test in diagnosing celiac disease. There were 800 people involved in the study and the prevalence of disease was 20%. The study found that the sensitivity and specificity of the test were 75% and 87.5%, respectively. Which of the follow statements is correct?

A) The positive predictive value is 80%.
B) The positive likelihood ratio is 6.0.
C) The false negative rate 12.5%.
D) The negative predictive value is 12.5%.

#2
04-13-2016
 USMLE Forums Scout Steps History: Not yet Posts: 12 Threads: 4 Thanked 0 Times in 0 Posts Reputation: 10
biostatic

#3
04-13-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 255 Threads: 26 Thanked 85 Times in 71 Posts Reputation: 95

Quote:
 Originally Posted by kumar1 answer is b?
i'll put the answer up in a few days.... i would like other people to have an attempt at the question first.

i posted the youtube lecture after being alerted to the fact that one obstacle for solving biostats questions is figuring out what the values are for A, B, C, D, etc,. so its not just an explanation to the question (which is not the purpose of the lecture), but a step-by-step guide to filling in the sample table and making various calculations of statistical measures. i hope that this lecture, The Sample Table: Walk-Through #1, will make that process a little easier for anyone who needs clarity on the topic.

Last edited by Kruno; 04-14-2016 at 07:12 AM.
#4
04-14-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 255 Threads: 26 Thanked 85 Times in 71 Posts Reputation: 95

Here's a two by two sample table completely filled in...

This is how the data was calculated based on the information provided...

A + C: The number of people with disease is 800 x 20% --> 160
B + D: The number of people without disease is 800 - 160 --> 640

A: The number of true positives [(A+C) x sensitivity] is 160 x 75% --> 120
C: The number of false negatives [(A+C) - true positives] is 160 - 120 --> 40
D: The number of true negatives [(B+D) x specificity] is 640 * 87.5% --> 560
B: The number of false positives [(B+D) - true negatives] is 640 - 560 --> 80

A + B: The total number of positive tests is 120 + 80 --> 200
C + D: The total number of negative tests is 40 + 560 --> 600
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Last edited by Kruno; 04-15-2016 at 01:43 AM.
#5
04-15-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 255 Threads: 26 Thanked 85 Times in 71 Posts Reputation: 95

Here is the answer to the question...

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