Specificity Sensitivity Question! - USMLE Forums
USMLE Forums Logo
USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
Home
USMLE Articles
USMLE News
USMLE Polls
USMLE Books
USMLE Apps
Go Back   USMLE Forums > USMLE Step 1 Forum

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


Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 03-25-2012
USMLE Forums Veteran
 
Steps History: 1+CK+CS
Posts: 257
Threads: 49
Thanked 75 Times in 50 Posts
Stats Specificity Sensitivity Question!

Assuming that mammography has a sensitivity of 90% and a specificity of 98% and that consecutive tests are independent, what is the probability that a woman with breast cancer will have a negative yearly screening mammogram for
two consecutive years?
a. 1/10
b. 2/10
c. 4/10
d. 1/100
e. 4/100
Reply With Quote Quick reply to this message



  #2  
Old 03-25-2012
USMLE Forums Guru
 
Steps History: Step 1 Only
Posts: 452
Threads: 11
Thanked 354 Times in 197 Posts
Reputation: 364
Default

D.........
Reply With Quote Quick reply to this message
  #3  
Old 03-26-2012
numbndumb's Avatar
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 529
Threads: 57
Thanked 282 Times in 150 Posts
Reputation: 292
Default D

They are asking about the probability of FALSE NEGATIVE results of mammogram for 2 consecutive events. FALSE NEGATIVE RATE = 1-seNsitivity = 1-0.9 = 0.1

Now the probability of having 2 false negative results in 2 consecutive yrs are independent events so u multiply the probabilities 0.1*0.1 or 1/10*1/10 =1/100 (option d)
Reply With Quote Quick reply to this message
The above post was thanked by:
Dr. Mexito (03-26-2012), excellence (05-18-2014), Hope2Pass (03-26-2012), Kruno (03-15-2016), taher (11-16-2013)
  #4  
Old 03-26-2012
USMLE Forums Veteran
 
Steps History: 1+CK+CS
Posts: 257
Threads: 49
Thanked 75 Times in 50 Posts
Default

yup the answer is D
Reply With Quote Quick reply to this message
  #5  
Old 11-16-2013
USMLE Forums Scout
 
Steps History: Not yet
Posts: 65
Threads: 9
Thanked 13 Times in 8 Posts
Reputation: 23
Default

Guys, I donot understand why do we have to multiply 1/10 of the first year with the 1/10 of next year as those two tests are independent. Why the answer is not just 1/10. Please explain?

If sensitivity is 90% means, out of 100 diseased 90 were correctly identified as diseased (True Positives) and 10% were missed (False negatives) &

If the specificity is 98% means, out of 100 healthy people 98 were correctly identified as normal (True negatives) and 2 were missed (False positives)

False Negative rate = False negatives/ No. of diseased = 10/100 = 1/10. I can get this far. But the multiplying thing. Elaborate please..........
Reply With Quote Quick reply to this message
  #6  
Old 08-10-2016
USMLE Forums Newbie
 
Steps History: Not yet
Posts: 3
Threads: 0
Thanked 0 Times in 0 Posts
Reputation: 10
Default

Quote:
Originally Posted by devareddy View Post
Guys, I donot understand why do we have to multiply 1/10 of the first year with the 1/10 of next year as those two tests are independent. Why the answer is not just 1/10. Please explain?

If sensitivity is 90% means, out of 100 diseased 90 were correctly identified as diseased (True Positives) and 10% were missed (False negatives) &

If the specificity is 98% means, out of 100 healthy people 98 were correctly identified as normal (True negatives) and 2 were missed (False positives)

False Negative rate = False negatives/ No. of diseased = 10/100 = 1/10. I can get this far. But the multiplying thing. Elaborate please..........

You have to multiply because we have to find the <probability that a woman with breast cancer will have a negative yearly screening mammogram for two consecutive year>. So she has to have 2 negative yearly screening mammograms for 2 consecutive years.
This type of probability is called COMPOUND PROBABILITY and it can be calculated by multiplying the porbabilities of each independent event.

So we have two independets events with a probability of 1/10 each one; we multiply 1/10*1/10 and we find 1/100 (answer D).
Reply With Quote Quick reply to this message



Reply

Tags
Biostatistics-Epidemiology, Step-1-Questions

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

Favorite USMLE Books
What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
Where you live. Leave blank if you don't want to tell.

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
Specificity Sensitivity Question! tyagee USMLE Step 1 Forum 3 03-09-2012 07:48 AM
sensitivity and specificity question? need explanation kemoo USMLE Step 1 Forum 5 03-30-2011 09:51 AM
parameters of sensitivity and specificity manphnx USMLE Step 1 Forum 6 01-31-2011 01:35 AM
What is the effect of the cut-off value on sensitivity/specificity/predictive values? zein1 USMLE Step 1 Forum 2 11-12-2010 05:28 AM
Sensitivity Specificity Equations step1 USMLE Step 1 Forum 4 12-26-2009 11:11 PM

RSS Feed
Find Us on Facebook
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

USMLE® & other trade marks belong to their respective owners, read full disclaimer
USMLE Forums created under Creative Commons 3.0 License. (2009-2014)