I've sent this once before but it got 'spammed' - so here is another go as it may be of general interest.
Ref to
Samia Alhabib
I'm afraid you've illustrated why it is difficult to explain what an odds ratio is and that it's meaning is context-sensitive.
Using your example, if we consider rates of diabetes in 200 males and 200 females, and we keep the odds ratio at 4.5 (as close as I could get to 4.5), you will see that as the odds of diabetes increase in each group the ratio of males/females with diabetes becomes smaller, making the explanation "male is 4.5 times more likely to have diabetes than female" increasingly incorrect.
Your explanation would be correct if you were talking about risk ratios (which are context-insensitive), but not odds ratios.
MALE FEMALE OR MALE/FEMALE
17/183 4/196 4.55 4.25
32/168
8/192 4.57 4
56/144 16/184 4.47 3.5
72/128 22/178 4.55 3.27
90/110 30/170 4.51 2.9
On Sun, 5 Feb 2012 08:10:18 -0800
s alhabib <
[log in to unmask]> wrote:
> 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 2
BN> 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 7
TJ> 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
