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#1
02-09-2014
 USMLE Forums Guru Steps History: Not yet Posts: 394 Threads: 128 Thanked 108 Times in 88 Posts Reputation: 118
Nnt = 1 / arr

New chemotherapy regimen is tested on 100 pts with lung cancer. 40 of pts are still alive after 1 yr.
Conventional chemotherapy is given to separate group, 500 pts. with lung cancer and 100 are alive after 1 yr..
Calculate NNT.

We know NNT = 1/ARR
IN DIT THEY SAID THAT FORMULA FOR ARR IS [ C/C+D ) - (A/A+B) ]

#2
02-09-2014
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,109 Threads: 31 Thanked 520 Times in 394 Posts Reputation: 539

Yes the above formula is good for 2x2 table.

Now we also know that ARR can be calculated if new know know different between the percentages of the control group and the conventional or standardized Group.

So the we treat alive people and so for the control group percentage alive is
40% and the conventional group is 20%
ARR 40-20=20%=0.2
NNT =1/0.2= 5.
U get same answer if u use mortality percentage.
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#3
02-09-2014
 USMLE Forums Scout Steps History: Not yet Posts: 13 Threads: 2 Thanked 6 Times in 4 Posts

About ARR---->think: there will be: 2 groups of people, one will have extra feature, and we want to compare the "RATE of something" in both groups.. lets see what we have here
we have group of Lung CA pts with NEW chemo and group of Lung CA pts with conventional chemo
so, the new chemo is the extra stuff. (extra feature can be a treatment.. but also can be an exposure to risk factor)..

and we want to compare the rate of death (mortality rate) in both groups (so we subtract rates to see the difference).
so:
New chemo: 60 (dead).......40(alive)====> so, of the 100 pts in the new chemo group, 60 died ==> mortality = 60/100= 60%

conv. chemo: 400 (dead).......100(alive)====> so, of the 500 pts in the conventional chemo group, 400 died==> mortality= 400/500 = 80%
Therefore, ARR = 80% - 60%= 20%

NNT= 1/0.20 = 10/2 = 5!
(it means: 5 lung CA pts need to be treated with new chemo to prevent one death related to Lung CA).
#4
02-10-2014
 USMLE Forums Scout Steps History: Not yet Posts: 44 Threads: 14 Thanked 8 Times in 8 Posts Reputation: 18

1) count ARR= New chemotherapy regimen - Conventional chemotherapy =
= 40/100 - 100/500 = 0.2

2) count NNT = 1/ARR = 1/0.2 = 5

 Tags Biostatistics-Epidemiology, Step-1-Questions

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