if we use for example a fair die or dice, however you spell that...what are the odds of rolling a 2 or chance or rolling a 2??? these are 2 different issues...the chance of the event , a '2' is 1/6....as only one '2' can be rolled out of 6 possible outcomes... or there is only one way a `2`can emerge out of 6 possible ways ....or 6 possible outcomes...1,2,3,4,5,or 6.....now the odds of rolling a 2 is 1/5 or .20 since it is the prob of an event /prob of the event not happening or a non event...or p
/ 1-p or (1/6) / (5/6) or .166
/ 1-.166 or .166/.8333 or 0.20.....am I correct in how I interpret this??? I still get screwed up trying to explain this but this is how i think of it...chance and odds here are 2 different things...and result in different outcomes...odds is the prob of the event / prob of the non event
so the chance or probability of rolling a '2' is 0.16 and the odds is 0.2.......
thus if 2/5 of patients in a study suffer DVT, the chance or probability is 0.4 yet the odds of their having DVT is
(2/5 ) / (3/5 ) = 0.4 /0.6 = 0.66 or 0.7 rounded off
chance or probability is 0.4 but odds or maybe use of the term risk is 0.7
if 1/5 of the patients in a study suffer a stroke, the odds of their having a stroke is (1/5) ÷ (4/5) or 0.20/0.80, or 0.25.
Best,
Paul E. Alexander
--- On Sun, 2/5/12, s alhabib <[log in to unmask]> wrote:
From: s alhabib <[log in to unmask]>
Subject: Re: Odds ratios explained
To: [log in to unmask]
Received: Sunday, February 5, 2012, 4:10 PM
Dear Simon:
An Odd ratio of 4.5, for example, in explaining the risk of developing diabetes in male than female is to say: male is 4.5 times more likely to have diabetes than female if they did not excercise...this is an a virtual example and does not imply any sort of evidence,
Samia Dr. Samia Alhabib, MD, MSc PHC, PG-Dip EBHC
Research Associate
Academic Unit of Primary health Care
Faculty of Medicine
University of Bristol,
Barley House, Oakfield Grove
Bristol, BS8 2BN
UK
From: Simon Hatcher <[log in to unmask]>
To: [log in to unmask]
Sent: Sunday, 5 February 2012, 15:35
Subject: Re: Odds ratios explained
In a previous post on this list someone asked the question
"how do you explain odds ratios to patients?" - which from memory no one ever answered. I understand odds ratios and I can draw 2x2 tables with the best of them however how do you include odds ratios in conversations with patients - in simple language what do odds ratios mean for patients?
Cheers
Simon
Associate Professor Simon Hatcher
Department of Psychological Medicine
Faculty of Medical and Health Sciences
The University of Auckland
New Zealand
________________________________________
From: Evidence based health (EBH) [[log in to unmask]] on behalf of k.hopayian [[log in to unmask]]
Sent: Monday, 6 February 2012 12:52 a.m.
To: [log in to unmask]
Subject: Re: Odds ratios explained
I am interested to know if the explanation below is any better than Wikipedia, it is the one I use with students and trainees.
Dr Kev (Kevork) Hopayian, MD FRCGP
Hon Sen Lecturer
Norwich Medical School
University of East Anglia
Norwich
NR4 7TJ
Making your practice evidence-based http://www.rcgp.org.uk/bookshop
The odds of something happening is the ratio of the probability of it happening to the probability of it not happening.
Let probability of an event = p (NB p is a proportion between 0 and 1)
Then the probability of an event not happening = 1–p
So odds = p/(1–p)
(In racing,
odds are usually given as the odds against something happening but we are dealing here with more lofty matters than the 2.15 at Epsom).
The odds ratio for two groups is simply the ratio of their odds.
Disorder present
Disorder absent
Exposed group
a
b
Comparison group
c
d
Look at the 2X2 table and see if you can follow this:
p in exposed group = a/(a+b)
Probability of event not happening in exposed group
= 1–p = 1– a/(a+b) = b/(a+b)
Odds in exposed group
= { a/(a+b)}/{ b/(a+b)}= a/b
Similarly, odds in control group
= c/d
So OR = {a/b}/{c/d} = ad/bc
If draw a line between the cells that multiply each other in the 2X2 table (a to d and b to c),
you may see why some people call the OR the cross test.
Three important things you need to know about ORs to get by in life without learning the calculation:
* An OR <1 means that fewer things happen in the exposed group than the comparison group (good when the thing is bad, e.g. a fall). An OR >1 means that more things happen in the exposed group than the comparison group (good when the thing is good, e.g. post-op pain relief, bad when the thing is bad, e.g. osteoporotic fracture). An OR =1 means no difference.
* ORs are not intuitive, e.g., OR = 2 does not mean the risk is doubled, it is not the same as RR (relative risk) = 2 except…
* …when the frequency of events (risk, event rate) is low, then OR is approximately the same as RR.
On 5 Feb 2012, at 10:51, Jane Hartley wrote:
Can anyone suggest an easy guide to odds ratios and other basic
stats functions?
A clinical friend has asked for some help with a dissertation and understanding source papers, this is not her area of expertise and she has been frightened off by the scholarly texts she has been directed too - I suspect her supervisors are not in their area of comfort with this either.
I have moved professionally away from supporting EBH and so am not up to date with user friendly articles. Dare I admit that I looked at the wikipedia which seemed comprehensive but impenetrable to the novice.
suggestions very welcome
Jane Hartley
