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#1
06-14-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 159 Threads: 52 Thanked 73 Times in 32 Posts Reputation: 83
Calculating the odds ratio

A study is conducted to study the effects of B Blockers on the onset of arrhythmias. 20 people out of 90 who took the b blockers developed arrhythmia and 30 out of 70 who did not take the drug developed arrhythmia.
What is the odds ratio of having an arrhythmia in the group taking the b blockers to the group who aren't.
A. 20X40/70X30
B. 20X70/30X40
C. 20X30/70X40
D. (20/50)/(70/110)
E. (20/90)/(30/70)

#2
06-14-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,331 Times in 891 Posts Reputation: 3341

2 by 2 table
……………..D…………no D
bb…………..20……………70.....total90
no bb………30…………...40....total70

so, odds of something = p / 1-p

"What is the odds ratio of having an arythmia in the gruop taking the b blocers to the gruop who arent."

odds having arythmia+bb = p / 1-p among Disease
= 20/30

odds not having arythmia+bb = p / 1-p among No Disease
= 70/40

OR = [20/30] / [70/40] = [20 * 40] / [30 * 70] which is the same as AD/CB

 The above post was thanked by: Charsmd (02-17-2015), Claus_CU (06-14-2011), leo lakhani (06-14-2011), Noa2013 (07-21-2014)
#3
06-14-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 159 Threads: 52 Thanked 73 Times in 32 Posts Reputation: 83

#4
06-14-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,331 Times in 891 Posts Reputation: 3341

Quote:
 Originally Posted by meka whats p in this? n wats 1-p ive never heard about this formula before:/
p = probability
1 - p = q = 1 - probability
and p + q must be equal to 1

odds = p / (1-p) is the mathematical definition
#5
06-14-2011
 USMLE Forums Guru Steps History: CK Only Posts: 475 Threads: 39 Thanked 366 Times in 176 Posts Reputation: 389

Quote:
 Originally Posted by bebix 2 by 2 table ……………..D…………no D bb…………..20……………70.....total90 no bb………30…………...40....total70 so, odds of something = p / 1-p "What is the odds ratio of having an arythmia in the gruop taking the b blocers to the gruop who arent." odds having arythmia+bb = p / 1-p among Disease = 20/30 odds not having arythmia+bb = p / 1-p among No Disease = 70/40 OR = [20/30] / [70/40] = [20 * 40] / [30 * 70] which is the same as AD/CB So, answer is A.
That's impressive! Thanks!
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 The above post was thanked by: bebix (06-14-2011)
#6
12-17-2013
 USMLE Forums Newbie Steps History: Not yet Posts: 4 Threads: 1 Thanked 1 Time in 1 Post Reputation: 11

Pneumonic

=Anno Domini/Before Christ

A will be both positive in 2 by 2 table and D as diagonal to A. Rest is piece of cake.. Good luck

 Tags Biostatistics-Epidemiology, Step-1-Questions

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