That is a Risk Ratio, not an Odds Ratio! It is stating that
(under the stated conditions) the probability of diabetes
for a male is 4.5 timres the probability for a female.
The following is probably beyond the "non-expert" reader
that Jane Hartley referred to in her original post, but it
is a note that I wrote a while ago to explain the differences
between the three standard comparators of risk:
Risk Difference, Risk Ratio (Relative Risk), Odds Ratio
Some mathematics is inevitable, but it was as simple as I
could make it without leaving out essential points of
comparison:
http://www.zen89632.zen.co.uk/R/TwoProportions/lambda_delta.pdf
Hoping this helps,
Ted.
On 05-Feb-2012 s alhabib 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
> Â_
> 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 = pÂ_ Â_ Â_ Â_ Â_ Â_ Â_ Â_ Â_ Â_ Â_ Â_ (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–pÂ_ Â_ Â_ Â_ Â_ Â_ = 1– a/(a+b)Â_ Â_ Â_ Â_ Â_ Â_ = b/(a+b)
> Odds in exposed group
> Â_ Â_ Â_ Â_ Â_ Â_ = { a/(a+b)}/{ b/(a+b)}= a/b
> Similarly, odds in control group
> = c/d
> So OR = {a/b}/{c/d}Â_ Â_ Â_ Â_ Â_ Â_ Â_ Â_ Â_ Â_ Â_ Â_ = 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:
>
> Â_ *Â_ 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 exposedÂ_ group than theÂ_ 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.
> Â_ *Â_ 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…
> Â_ *Â_ …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.Â_ Dare I admit that I looked at the wikipedia
> which seemed comprehensive but impenetrable to the novice.
>
> suggestions very welcome
>
> Jane Hartley
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E-Mail: (Ted Harding) <[log in to unmask]>
Date: 05-Feb-2012 Time: 17:09:14
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