Conditional Probability Test - USMLE Forums
USMLE Forums Logo
USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
USMLE Articles
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

Thread Tools Search this Thread Display Modes
Old 03-12-2013
USMLE Forums Guru
Steps History: 1+CK+CS
Posts: 333
Threads: 162
Thanked 284 Times in 147 Posts
Reputation: 294
Question Conditional Probability Test

The table shows survival information for patients who had operation for a particular form of cancer (See the attached).
Based on this information if a patient survives for one year, which of the following is the probability that he or she will survive for 4 years

a) 0.875 - 0.8 * 0.9

b) 0.8*0.875*0.9*0.9

c) 0.875*0.9

d) 0.875*0.9*0.9

e) 0.9

f) 0.9 - 0.875
Attached Thumbnails
Conditional Probability Test-untitled.jpg  
Reply With Quote Quick reply to this message

Old 03-12-2013
USMLE Forums Scout
Steps History: ---
Posts: 75
Threads: 12
Thanked 63 Times in 33 Posts
Reputation: 73

ans. D) 0.875 X 0.9 X 0.9


Short explanation:

Multiply the "% patients surviving this interval" AFTER then ONE year that he already survive...

After the ONE year meaning:
1-2 years (.875),
2-3 years (0.9),
3-4 years ( 0.9)

D) 0.875 X 0.9 X 0.9


Long explanations (if you need further clarifications):

Conditional Probability:....

If given that the pt survived say like 1 yr( or 2 yrs etc.) and asking for probability of surviving certain # of yrs (e.g 2 yrs or 3 yrs etc.).

and the data is given in one of these two ways:


0--1 yr = 80 %
0--2 yr = 60 %
0--3 yr = 40 %
0--4 yr = 20 %

say like if pt survived 1 yr what would be the probability--?(in this case u DIVIDE probability)

DIVIDE by (prob.of # yr pt already survived e.g. 1 yr)

probability of surviving 2 yrs.......60/80 =75%
probability of surviving 3 yrs.......40/80 =50%
probability of surviving 4 yrs.......20/80 =25%

if the pt survived say like 2 yrs and probability of surviving 4 yrs?
20/60 =33 %


0--1 yr = 80%
1--2 yr = 60%
2--3 yr = 40%
3--4 yr = 20%

say like if pt survived 1 yr ,what would be the probability? (in this case MULTIPLY probability.)

Note: Multiply probabilities in between(SKIPPING the survived yr probability in this case 80%)

probability of surviving 3 yrs = 60 times 40
probability of surviving 4 yrs = 60 times 40 times 20

If asked probability of surviving straight say like 4 yrs(from the day one without condition of already survived certain # of yr) = multiply all probabilities 80 times 60 times 40 times 20

answer is D) 0.875 times 0.9 times 0.9

B- would have been the answer if asked straight 4 yr survival

C- is the probability of surviving 3 yrs once the pt already survived for 1 yr
Reply With Quote Quick reply to this message
The above post was thanked by:
Doctor Ali (03-12-2013), Evergreen (03-12-2013)


Quick Reply

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:
Please enter a password for your user account. Note that passwords are case-sensitive.
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.
Where you live. Leave blank if you don't want to tell.


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
The probability of Positive Test! nishaa USMLE Step 1 Forum 20 10-21-2012 04:27 PM
probability question sruthi USMLE Step 1 Forum 2 08-08-2012 12:19 AM
the probability USMLE-Syndrome USMLE Step 1 Forum 4 02-08-2012 12:05 AM
Conditional Probability after surviving the first year? usmleguy USMLE Step 1 Forum 5 11-13-2011 10:08 PM
The probability of having Alzheimer’s disease given the test is positive is... bebix USMLE Step 2 CK Forum 11 05-27-2011 01:22 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)