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 USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam

#1
03-02-2012
 USMLE Forums Addict Steps History: Not yet Posts: 175 Threads: 18 Thanked 63 Times in 48 Posts Reputation: 73
Weird Statistics

Investigators are studying the use of a new laboratory test to identify patients with a particular disease. The table
below summarizes the results of initial research involving 200 subjects.
Disease
Present Absent
Test Result
Positive 60 40 100
Negative 20 80 100
80 120 200
Which of the following is the approximate sensitivity of a positive test result?
(A) 0.30
(B) 0.33
(C) 0.60
(D) 0.67
(E) 0.75

I marked C but the answer says E

#2
03-02-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,406 Threads: 118 Thanked 642 Times in 407 Posts Reputation: 652

pretty simple man.

E is correct.

60/60+20 = 0.75
__________________
"inflammable means flammable!? What a country."
 The above post was thanked by: DocSikorski (03-02-2012)
#3
03-02-2012
 USMLE Forums Addict Steps History: Not yet Posts: 175 Threads: 18 Thanked 63 Times in 48 Posts Reputation: 73

Disease
Present Absent
Test
+ 60 TP 40FP

- 20 FN 80TN

hmmm seems that i was in a hurry doing blocks
hoping i dont repeat careless mistakes in the real exam
#4
03-02-2012
 USMLE Forums Scout Steps History: --- Posts: 46 Threads: 1 Thanked 27 Times in 18 Posts Reputation: 37

To avoid mistakes, draw yourself a table. You'd get more accurate results!
I learned this through the kaplan's biostats video's.
#5
03-02-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 653 Threads: 47 Thanked 408 Times in 249 Posts Reputation: 418

Easy question, let me tell you why.

You just have to follow some simple rules.

1. Always draw your 2x2 table.
and add the data they give you to it.

then.

2. Remember this table and for the formulas this is the most important rule: Trues always on top!!! For example, you want sensitivity, first colum you have TP and FN, the forumla is TP/ TP+FN, you see true is on top, then for specificity, next column, and TN is on top. For PPV NPV again, use the table, and trues on top.

Always do your own table, they might give you a table done, but mixed.
 The above post was thanked by: mizzy (03-02-2012), ras9191 (03-29-2012)

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