Absolute risk reduction versus Relative risk reduction - USMLE Forums
 USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
 Home USMLE Articles USMLE News USMLE Polls USMLE Books USMLE Apps
 USMLE Forums Absolute risk reduction versus Relative risk reduction
 Register FAQs Members List Search Today's Posts Mark Forums Read

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

#1
01-19-2013
 USMLE Forums Addict Steps History: 1 + CK Posts: 150 Threads: 37 Thanked 188 Times in 61 Posts Reputation: 198
Absolute risk reduction versus Relative risk reduction

160 people in placebo , 80 died .
200 people in Drug X , 50 died .
what % of deaths were prevented by DRug X ?

#2
01-20-2013
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 236 Threads: 12 Thanked 194 Times in 98 Posts Reputation: 206

Number needed to treat(NNT) in this case prevent deaths=1/ARR=50%
#3
01-20-2013
 USMLE Forums Scout Steps History: Not yet Posts: 57 Threads: 13 Thanked 17 Times in 15 Posts Reputation: 27

Quote:
 Originally Posted by doc.mav Number needed to treat(NNT) in this case prevent deaths=1/ARR=50%
doc .mav could you please explain how you got 50% ....i really get confused with this NNT ....ARR for this case is 25% ..is it right ... ??

#4
01-20-2013
 USMLE Forums Addict Steps History: 1 + CS Posts: 142 Threads: 33 Thanked 101 Times in 29 Posts Reputation: 111

Quote:
 Originally Posted by doc.mav Number needed to treat(NNT) in this case prevent deaths=1/ARR=50%

NNT would not be given in a percentage.

This question is just asking attributable risk reduction.
(80/160)-(50/200)= 25%
With the drug we have prevented 25% of the fatal cases from occurring.
 The above post was thanked by: Awwad (03-17-2013)
#5
11-11-2013
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 515 Threads: 88 Thanked 508 Times in 232 Posts Reputation: 518

Quote:
 Originally Posted by SOBE223 NNT would not be given in a percentage. This question is just asking attributable risk reduction. (80/160)-(50/200)= 25% With the drug we have prevented 25% of the fatal cases from occurring.
To be correct answer you should divide by 80/160, so the correct answer is 50%, not 25%.
#6
11-12-2013
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 16 Threads: 4 Thanked 4 Times in 2 Posts Reputation: 14

Quote:
 Originally Posted by ag2011n To be correct answer you should divide by 80/160, so the correct answer is 50%, not 25%.
ARR=(80/160)-(50/200)=(1/2)-(1/4)=1/4=25%
Number needed to treat(NNT)=1/ARR=4
#7
11-12-2013
 USMLE Forums Scout Steps History: Not yet Posts: 45 Threads: 9 Thanked 12 Times in 5 Posts Reputation: 22

25 %................................................. ....
#8
11-22-2013
 USMLE Forums Scout Steps History: Not yet Posts: 11 Threads: 0 Thanked 6 Times in 4 Posts Reputation: 16

absolute risk reduction= subtract the risk ratio of drugX from risk ratio of placebo.

ARR= 80/160 - 50/200 = 0.5 - 0.25 = 0.25 = 25%

so, drug X reduced the absolute risk of death by 25% in the population

relative risk reduction = divide the risk ratio of placebo group with risk ratio of drugX

RRR= 50/200 / 80/160 = 0.5/0.25 = 2

so, relative risk of death with drug X is 2 times less likely than in the placebo group. Or
in other words, placebo group has 2 times more risk of death than the drug X.
#9
11-22-2013
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 41 Threads: 20 Thanked 24 Times in 10 Posts Reputation: 34
RRR

Quote:
 Originally Posted by organo14 absolute risk reduction= subtract the risk ratio of drugX from risk ratio of placebo. ARR= 80/160 - 50/200 = 0.5 - 0.25 = 0.25 = 25% so, drug X reduced the absolute risk of death by 25% in the population relative risk reduction = divide the risk ratio of placebo group with risk ratio of drugX RRR= 50/200 / 80/160 = 0.5/0.25 = 2 so, relative risk of death with drug X is 2 times less likely than in the placebo group. Or in other words, placebo group has 2 times more risk of death than the drug X.

RRR= ARR/Control Group .25/.5= .5 = 50 percent RRR
Correct me if im wrong
#10
11-23-2013
 USMLE Forums Scout Steps History: Not yet Posts: 11 Threads: 0 Thanked 6 Times in 4 Posts Reputation: 16

Quote:
 Originally Posted by Oscaryasser RRR= ARR/Control Group .25/.5= .5 = 50 percent RRR Correct me if im wrong
well, i used the formula of relative risk, which tells not only the increased risk in a group but also a reduced risk in the other group, depending upon what is being asked.

so, i used the formula telling that placebo group has 2 times more chances of deaths than in the drugX group.

so, i can aslo reframe it as, drugX has 2 times less chances of deaths than in placebo group.
' 2 times less' means one half chances (0.5),
so drugX has 0.5 chances of deaths than in placebo group ( as you calculated).

the only diference is in the method, you directly put it and i indirectly.

your way is very useful if we knew the ARR. i hadn't come across it.

thankyou
 The above post was thanked by: Oscaryasser (11-23-2013)

 Tags Biostatistics-Epidemiology

Message:
Options

## Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
User Name:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
 AMG IMG ---
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.

 Not yet Step 1 Only CK Only CS Only 1 + CK 1 + CS 1+CK+CS CK+CS 1+CK+CS+3 ---
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.