BioStat Question for Today - 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 01-29-2014
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 592
Threads: 92
Thanked 198 Times in 129 Posts
Reputation: 208
Send a message via Skype™ to drpisho
Question BioStat Question for Today

Good morning everyone, i leave you with a question that i found hard but is good cause that way we master biostat.
if anyone has more questions please add the links or put them here cause id like to practice since im terrible at biostat
good day to everyone!!

A new screening test for the development of mitral regurgitation in the setting of rheumatic fever is created. A study of 1000 patients with a history of s.pyogenes infection and a diagnosis of rheumatic fever is performed using this test which has 90% sensitivity and 85% specificity, the prevalence of mitral regurgitation in this population is estimated to be 40%.
what is the positive predictive value of this test?

A) 8%
B) 20%
C) 80%
D) 85%
E) 93%
__________________
Will. Power. Faith. MATCHED IM 2016!
Reply With Quote Quick reply to this message
The above post was thanked by:
Borzoo400 (02-08-2014)



  #2  
Old 01-29-2014
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,109
Threads: 31
Thanked 521 Times in 394 Posts
Reputation: 540
Default

Quote:
Originally Posted by drpisho View Post
Good morning everyone, i leave you with a question that i found hard but is good cause that way we master biostat.
if anyone has more questions please add the links or put them here cause id like to practice since im terrible at biostat
good day to everyone!!

A new screening test for the development of mitral regurgitation in the setting of rheumatic fever is created. A study of 1000 patients with a history of s.pyogenes infection and a diagnosis of rheumatic fever is performed using this test which has 90% sensitivity and 85% specificity, the prevalence of mitral regurgitation in this population is estimated to be 40%.
what is the positive predictive value of this test?

A) 8%
B) 20%
C) 80%
D) 85%
E) 93%

Diseased poeple = 40% of 1000 = 400
Therefore.

.................... ..... MR+. ............ MR-

Positive result ...... 360. ............ 90

Neg result. ........ 40. ............. 510


NPV. 360/(360+90)= 80%


Ans: ppv= 80%
__________________
In USMLE, “Any fool can know. The point is to understand.”

Last edited by nsesereso; 01-29-2014 at 07:38 AM.
Reply With Quote Quick reply to this message
The above post was thanked by:
Borzoo400 (02-07-2014)
  #3  
Old 01-29-2014
USMLE Forums Scout
 
Steps History: Not yet
Posts: 29
Threads: 6
Thanked 3 Times in 3 Posts
Reputation: 13
Default

i think is 85 % but i didnt use the prevelance in my calculations. i used 2x2 box a= 900 b=150 c=100 d=850 ppv= a/(a+b)
Reply With Quote Quick reply to this message
 
  #4  
Old 01-29-2014
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,109
Threads: 31
Thanked 521 Times in 394 Posts
Reputation: 540
Default

Quote:
Originally Posted by dr_marini View Post
i think is 85 % but i didnt use the prevelance in my calculations. i used 2x2 box a= 900 b=150 c=100 d=850 ppv= a/(a+b)
The prevalence given is not for fun my friend.
__________________
In USMLE, “Any fool can know. The point is to understand.”
Reply With Quote Quick reply to this message
  #5  
Old 01-29-2014
USMLE Forums Scout
 
Steps History: 1 + CS
Posts: 17
Threads: 1
Thanked 4 Times in 3 Posts
Reputation: 14
Default

Yup, I agree with nsesereso. Answer is 80%. You have to take the prevalence into account.
Reply With Quote Quick reply to this message
The above post was thanked by:
dr_marini (01-29-2014)
  #6  
Old 02-07-2014
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 592
Threads: 92
Thanked 198 Times in 129 Posts
Reputation: 208
Send a message via Skype™ to drpisho
Thumbs Up

nsesereso and Doc889 very good!! please pass me some of your
biostat knowledge!! this question made me sweat lol.
thanks for the explanation also.
__________________
Will. Power. Faith. MATCHED IM 2016!
Reply With Quote Quick reply to this message
  #7  
Old 02-08-2014
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,109
Threads: 31
Thanked 521 Times in 394 Posts
Reputation: 540
Default

Quote:
Originally Posted by drpisho View Post
nsesereso and Doc889 very good!! please pass me some of your
biostat knowledge!! this question made me sweat lol.
thanks for the explanation also.
Then u also pass some of the other subjects to me. Am sweating too my friend, it's not easy.
__________________
In USMLE, “Any fool can know. The point is to understand.”
Reply With Quote Quick reply to this message
  #8  
Old 04-01-2016
USMLE Forums Veteran
 
Steps History: 1+CK+CS+3
Posts: 285
Threads: 32
Thanked 86 Times in 71 Posts
Reputation: 96
Default

Here is a youtube video with an animated explanation on how to calculate the positive predictive value... https://www.youtube.com/watch?v=Xhm2Ida053Y

The positive predictive value is the proportion of positive tests that occur in individuals that have the condition. In order to solve this question you need to know the number of true positives (i.e., people with mitral regurgitation who tested positive) and the total number of positive tests. These values are not given, but can be determined from the information provided.

Sample Table

----------------------------------Disease------------------------------
-------------------------Present----------Absent----------------------
Test Positive ----------A (360)----------B (90)-----------A+B (450)
Test Negative ---------C (40)-----------D (510)---------C+D (550)
------------------------A+C (400)-------B+D (600)-------------(1000)

How To Fill The Table

A + C: The number of people with mitral regurgitation is 40% * 1000, which is 400.
B + D: The number of people without mitral regurgitation is 1000 - 400, which is 600.

A: The number of true positives [(A+C) * sensitivity] is 600 * 90%, which is 360.
D: The number of true negatives [(B+D) * specificity] is 400 * 85%, which is 510.
B: The number of false positives [(B+D)-D] is 600 - 510, which is 90.

[B]A + B: The total number of positive tests is 360 + 90, which is 450.

Answer

The positive predictive value is calculated by dividing the number of true positives by the total number of positive tests.
Positive Predictive Value [A/(A+B)] = 360/450....... 80%
Attached Thumbnails
BioStat Question for Today-big-title-cards.jpg   BioStat Question for Today-blue-punnet-cards.jpg   BioStat Question for Today-info-cards.jpg  
__________________
Check out my
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
channel on youtube.

Last edited by Kruno; 04-01-2016 at 02:53 AM. Reason: spelling error
Reply With Quote Quick reply to this message



Reply

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
Biostat question mean,median and mode vnagubandi USMLE Step 1 Forum 5 01-03-2013 10:55 AM
Biostat medhelp USMLE Step 1 Forum 2 11-03-2012 02:18 PM
Prevalence, Sensitivity, Specificity Biostat Question! sonu.agarwall USMLE Step 2 CK Forum 1 07-31-2012 08:05 AM
biostat confidence interval question? dr.dhruvdesai USMLE Step 1 Forum 5 07-18-2011 11:25 AM
biostat Hardy weinberg question dr.dhruvdesai USMLE Step 1 Forum 3 07-18-2011 02:23 AM

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)