Biostatistics #18 - Screening tests - 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 06-10-2011
bebix's Avatar
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,357
Threads: 194
Thanked 3,262 Times in 881 Posts
Reputation: 3272
Stats Biostatistics #18 - Screening tests

A screening test for breast cancer was administered to 400 women with biopsy-proven breast cancer and to 400 women without breast cancer. The test results were positive for 100 of the proven cases and 50 of the normal women.

1. What is the probability that the patient has the disease, given a positive test?
a. 88%
b. 67%
c. 25%
d. 33%
e. 12%

2. What is the probability that the test is positive, given that the patient has the disease?
a. 88%
b. 67%
c. 25%
d. 33%
e. 12%

3. What is the probability that the test is positive, given that the patient don't have the disease?
a. 88%
b. 67%
c. 25%
d. 33%
e. 12%
Reply With Quote Quick reply to this message
The above post was thanked by:
aknz (10-08-2011), Al-Saoudi (06-10-2011), excellence (05-18-2014), pass7 (06-10-2011), Rsadhu (01-13-2015), rulz (10-06-2011)



  #2  
Old 06-10-2011
Dr_Laura's Avatar
USMLE Forums Guru
 
Steps History: 1+CK+CS
Posts: 499
Threads: 85
Thanked 529 Times in 222 Posts
Reputation: 539
Default

1. D
2. C
3. C

I'm really having a hard time with these... Thanks for all the questions!
__________________
YouTube:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Facebook:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Reply With Quote Quick reply to this message
The above post was thanked by:
bebix (06-10-2011)
  #3  
Old 06-10-2011
pass7's Avatar
USMLE Forums Guru
 
Steps History: 1+CK+CS+3
Posts: 406
Threads: 30
Thanked 141 Times in 109 Posts
Reputation: 151
Default

i dont know!!
Reply With Quote Quick reply to this message
 
  #4  
Old 06-10-2011
bebix's Avatar
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,357
Threads: 194
Thanked 3,262 Times in 881 Posts
Reputation: 3272
Thumbs Up

Quote:
Originally Posted by pass7 View Post
i dont know!!
@pass7...just relax, take a deep breath and try again...
Reply With Quote Quick reply to this message
  #5  
Old 06-10-2011
USMLE Forums Scout
 
Steps History: Not yet
Posts: 34
Threads: 9
Thanked 14 Times in 11 Posts
Reputation: 24
Default

answer may
b.
c.
d.
Reply With Quote Quick reply to this message
The above post was thanked by:
bebix (06-10-2011)
  #6  
Old 06-10-2011
dr.Irina's Avatar
USMLE Forums Veteran
 
Steps History: 1 + CK
Posts: 229
Threads: 6
Thanked 69 Times in 61 Posts
Reputation: 79
Default

1. A
2. c
3. e
Reply With Quote Quick reply to this message
The above post was thanked by:
bebix (06-10-2011)
  #7  
Old 06-10-2011
USMLE Forums Guru
 
Steps History: Not yet
Posts: 324
Threads: 70
Thanked 238 Times in 144 Posts
Reputation: 248
Default

Quote:
Originally Posted by bebix View Post
a screening test for breast cancer was administered to 400 women with biopsy-proven breast cancer and to 400 women without breast cancer. The test results were positive for 100 of the proven cases and 50 of the normal women.

1. What is the probability that the patient has the disease, given a positive test?
a. 88%
b. 67%
c. 25%
d. 33%
e. 12%

2. What is the probability that the test is positive, given that the patient has the disease?
a. 88%
b. 67%
c. 25%
d. 33%
e. 12%

3. What is the probability that the test is positive, given that the patient don't have the disease?
a. 88%
b. 67%
c. 25%
d. 33%
e. 12%
1.b
2.c
3.a
Reply With Quote Quick reply to this message
The above post was thanked by:
bebix (06-10-2011)
  #8  
Old 06-10-2011
USMLE Forums Newbie
 
Steps History: Not yet
Posts: 2
Threads: 0
Thanked 3 Times in 2 Posts
Reputation: 13
Default nice

1= positive pred value= 66.67%
2=sensitivity= 25%
3= false positive rate= 1-specificity= 1-.12=88%
Reply With Quote Quick reply to this message
The above post was thanked by:
bebix (06-10-2011)
  #9  
Old 06-10-2011
bebix's Avatar
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,357
Threads: 194
Thanked 3,262 Times in 881 Posts
Reputation: 3272
Correct Answer correct answer

Correct answers:
1-B
2-C
3-E


Gold Standard (GS) = biopsy (Disease = D)

First, let's create a 2x2 table:

..........Disease(+)....Disease(-)
Test(+).....100...........50.......|TOTAL = 150
Test(-).....300..........350.......|TOTAL = 650
----------------------------------------------
...............400..........400.......|TOTAL = 800

1.-What is the probability that the patient has the disease, given a positive test? = PPV => P(D|+)

P(D|+)
- numerator Disease AND test (+)
- denominator ALL test (+)

= 100/150 = 2/3 or 67%


2.-What is the probability that the test is positive, given that the patient has the disease? = Sensitivity => P(+|D)

P(+|D)
- numerator test (+) AND Disease
- denominator ALL Disease(+)

= 100/400 = 1/4 or 25%

3.-What is the probability that the test is positive, given that the patient don't have the disease? = 1-Specificity => P(+|no D)

P(+|no D)
- numerator test (+) AND NO Disease
- denominator ALL Disease(-)

= 50/400 = 1/8 or 12.5%
Reply With Quote Quick reply to this message
The above post was thanked by:
aknz (10-08-2011), Dr_Laura (06-10-2011), drortho (11-23-2011), excellence (05-18-2014), nevillenic (06-10-2011), pass7 (06-10-2011), steps_8186 (04-11-2012)
  #10  
Old 06-10-2011
Dr_Laura's Avatar
USMLE Forums Guru
 
Steps History: 1+CK+CS
Posts: 499
Threads: 85
Thanked 529 Times in 222 Posts
Reputation: 539
Default

Thanks so much! This has helped me a lot
__________________
YouTube:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Facebook:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Reply With Quote Quick reply to this message
The above post was thanked by:
bebix (06-10-2011)
  #11  
Old 06-10-2011
USMLE Forums Scout
 
Steps History: Not yet
Posts: 34
Threads: 9
Thanked 14 Times in 11 Posts
Reputation: 24
Default

Quote:
Originally Posted by bebix View Post
Correct answers:
1-B
2-C
3-E


Gold Standard (GS) = biopsy (Disease = D)

First, let's create a 2x2 table:

..........Disease(+)....Disease(-)
Test(+).....100...........50.......|TOTAL = 150
Test(-).....300..........350.......|TOTAL = 650
----------------------------------------------
...............400..........400.......|TOTAL = 800

1.-What is the probability that the patient has the disease, given a positive test? = PPV => P(D|+)

P(D|+)
- numerator Disease AND test (+)
- denominator ALL test (+)

= 100/150 = 2/3 or 67%


2.-What is the probability that the test is positive, given that the patient has the disease? = Sensitivity => P(+|D)

P(+|D)
- numerator test (+) AND Disease
- denominator ALL Disease(+)

= 100/400 = 1/4 or 25%

3.-What is the probability that the test is positive, given that the patient don't have the disease? = 1-Specificity => P(+|no D)

P(+|no D)
- numerator test (+) AND NO Disease
- denominator ALL Disease(-)

= 50/400 = 1/8 or 12.5%
ohh..thnks....i was just near to answer..only 3rd was wrong....just slip in question to understand and made it wrong..hope this type of sily mistake will not occur in real exam.....thnks bebix for such good question..
Reply With Quote Quick reply to this message
The above post was thanked by:
bebix (06-10-2011)
  #12  
Old 06-10-2011
USMLE Forums Guru
 
Steps History: 1 + CS
Posts: 438
Threads: 111
Thanked 232 Times in 132 Posts
Reputation: 242
Smile

#3 question is 88%.
Trues are always on top.
350/400=88%
Reply With Quote Quick reply to this message
  #13  
Old 06-10-2011
USMLE Forums Scout
 
Steps History: Not yet
Posts: 34
Threads: 9
Thanked 14 Times in 11 Posts
Reputation: 24
Default

Quote:
Originally Posted by evavar View Post
#3 question is 88%.
Trues are always on top.
350/400=88%
no evavar...positives are on top......and true positives are on top and true negatives are on bottom..
Reply With Quote Quick reply to this message
  #14  
Old 06-10-2011
bebix's Avatar
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,357
Threads: 194
Thanked 3,262 Times in 881 Posts
Reputation: 3272
Smile

Quote:
Originally Posted by evavar View Post
#3 question is 88%.
Trues are always on top.
350/400=88%
Specificity = Probability of a NEGATIVE test given the person DON'T have the Disease

P(-|no D) = 350/400 = 88%

1 - Specificity = Probability of a POSITIVE test given the person DON'T have the Disease

P(+|no D) = 50/400 = 1/8 or 12.5%

I never use true positive, true negative....this is just a conditional probability (something GIVEN something else)...so, the "something else" MUST be in the DENOMINATOR.

Hope this help
Reply With Quote Quick reply to this message
The above post was thanked by:
pass7 (06-10-2011)
  #15  
Old 06-10-2011
USMLE Forums Guru
 
Steps History: Not yet
Posts: 298
Threads: 45
Thanked 718 Times in 210 Posts
Reputation: 728
Default

Bebix, you are right about taking a deep breath.

On these questions, make sure to read the question carefully and understand what is being asked.

In these questions, we are given the exact parameters we need to answer the questions without having any knowledge of sensitivity, specificity, PPV, NPV, etc.

1. Of the people who tested positive (150 tested positive), how many had the disease (100), therefore 100/150 = 2/3 = 67%

2. Given the patient has the disease (400 people fall into this category, how many tested positive (100 with the disease tested positive). Therefore 100/400 = 25%

3. Given the patient doesnt have the disease (400 don't), how many tested positive (50 tested positive when they didnt have the disease). Therefore 50/400 = 12%

They are going to try to trick us with the wording on these types of questions so take the extra 30 seconds to reword/rephrase the question so that it makes more sense to you.
Reply With Quote Quick reply to this message
The above post was thanked by:
excellence (05-18-2014)



  #16  
Old 06-10-2011
bebix's Avatar
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,357
Threads: 194
Thanked 3,262 Times in 881 Posts
Reputation: 3272
Thumbs Up

Quote:
Originally Posted by apx85 View Post
Bebix, you are right about taking a deep breath.

On these questions, make sure to read the question carefully and understand what is being asked.

In these questions, we are given the exact parameters we need to answer the questions without having any knowledge of sensitivity, specificity, PPV, NPV, etc.

1. Of the people who tested positive (150 tested positive), how many had the disease (100), therefore 100/150 = 2/3 = 67%

2. Given the patient has the disease (400 people fall into this category, how many tested positive (100 with the disease tested positive). Therefore 100/400 = 25%

3. Given the patient doesnt have the disease (400 don't), how many tested positive (50 tested positive when they didnt have the disease). Therefore 50/400 = 12%

They are going to try to trick us with the wording on these types of questions so take the extra 30 seconds to reword/rephrase the question so that it makes more sense to you.
yup, math never change...I think is better to understand the concept, rather than memorize weird formulas...here is a new one

Biostatistics #19 - Screening tests (part b)
Reply With Quote Quick reply to this message
The above post was thanked by:
rulz (10-06-2011)
  #17  
Old 01-12-2015
dr mumble's Avatar
USMLE Forums Scout
 
Steps History: CK+CS
Posts: 87
Threads: 22
Thanked 12 Times in 12 Posts
Reputation: 22
Default

Quote:
Originally Posted by bebix View Post
A screening test for breast cancer was administered to 400 women with biopsy-proven breast cancer and to 400 women without breast cancer. The test results were positive for 100 of the proven cases and 50 of the normal women.

1. What is the probability that the patient has the disease, given a positive test?
a. 88%
b. 67%
c. 25%
d. 33%
e. 12%

2. What is the probability that the test is positive, given that the patient has the disease?
a. 88%
b. 67%
c. 25%
d. 33%
e. 12%

3. What is the probability that the test is positive, given that the patient don't have the disease?
a. 88%
b. 67%
c. 25%
d. 33%
e. 12%


i think last one is tricky ,,
he is asking about false positve rate
FP rate= 1 - specificity
so 1st we find out specificity , that's trues are on top divided by everything in that group
so specificity= 350/400 = 88%

now FPrate is =1-88=12%


so i think answer is clear
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
NBME tests! ferdenand USMLE Step 2 CK Forum 2 10-16-2013 10:22 AM
Biostatistics #14 - Screening test bebix USMLE Step 1 Forum 6 06-06-2011 07:38 PM
Biostatistics #3 - Screening test bebix USMLE Step 1 Forum 9 06-06-2011 02:17 PM
Step 2 CK Self Assessment Tests meanmr.mustard USMLE Step 2 CK Forum 1 05-26-2011 11:08 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)