Hi Amy
Be great to see what these diagrams with circles look like. Can you post them as some sort of attachment?
Cheers
Simon
Associate Professor Simon Hatcher
Department of Psychological Medicine
Faculty of Medical and Health Sciences
The University of Auckland
New Zealand
________________________________________
From: Dr. Amy Price [[log in to unmask]]
Sent: Monday, 6 February 2012 6:41 a.m.
To: Simon Hatcher; [log in to unmask]
Subject: RE: Odds ratios explained
Hi Simon
I use the diagrams with the circles like they use for prostrate cancer and
advice for statins. Actually if anyone want to know where one of these
exists for breast cancer and BC screening I would be grateful This way it is
concrete, takes no abstract thought and is free of maths. I also do this for
myself so I can see what I am looking at. Patients don't care about the
population as much as they care about what is their personal risk and they
frequently get Odds wrong, turning them into fractions etc.
Best Amy
-----Original Message-----
From: Evidence based health (EBH)
[mailto:[log in to unmask]] On Behalf Of Simon Hatcher
Sent: 05 February 2012 10:35 AM
To: [log in to unmask]
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
