Dear all
I want to give my thanks to everyone who has contributed to the discussion
about odds ratios and relative risk. As a researcher in healthcare, I find
this sort of exchange one of the really useful and educational aspects of
belonging to this mailbase. Please keep this useful information coming!
With best wishes
Roy
-----Original Message-----
From: Eric Harvey <[log in to unmask]>To:
[log in to unmask] <[log in to unmask]>
Date: 25 February 1999 16:09
Subject: Re: Odds Ratios vs Relative Risk
>Andrew,
>
>Sorry that my use of terms was unclear. Relative risk requires an estimate
>of prevalence in the total population (to use your term). Odds ratios can
>be calculated using only study population prevalence. Odds ratios
>approximate relative risk only if the prevalence in the total population is
>very low.
>
>Relative risk can be determined if a sufficiently large sample is taken
from
>the total population, no matter what the true prevalence is.
>
>Odds ratios were originally calculated when analyzing retrospective data
>(case-control studies), whereas relative risk can be calculated from
>prospective data (RCTs) if the sample is sufficiently representative of the
>population. Since causality cannot be demonstrated with retrospective
data,
>the OR is a less accurate predictor of future events than is RR. This is
>the usual way I distinguish between the two.
>
>I apologize to any reader who feels I've made the destinctions too
>simplistic.
>
>______________________________________________
>
>Eric Harvey, PharmD, MBA, BCPS
>Child Health Institute, Evidence-Based Medicine Project
>146 North Canal Street, Suite 300
>Seattle, WA 98103-8652
>
>University of Washington Campus Mailbox: 358853
>
>office: (206) 616-1203
>fax: (206) 543-5318
>e-mail: [log in to unmask]
>____________________________________________
>
>-----Original Message-----
>From: Andrew Jull <[log in to unmask]>
>To: [log in to unmask]
><[log in to unmask]>
>Date: Wednesday, February 24, 1999 6:03 PM
>Subject: RE: Odds Ratios vs Relative Risk
>
>
>>Hi Eric
>>
>>I'm not sure I understand this - would you not have access to the study's
>>population prevalence (ie you need to know both the number of events and
>the
>>number without events to calculate the odds) when calculating the OR? Or
>are
>>you referring to knowledge of prevalence in the total population, not just
>>the study population? If this former should be the case, then should not
>>most studies be using ORs?
>>
>>regards
>>Andrew
>>
>>> -----Original Message-----
>>> From: Eric Harvey [SMTP:[log in to unmask]]
>>> Sent: Thursday, 25 February 1999 09:29
>>> To: [log in to unmask]
>>> Subject: Re: Odds Ratios vs Relative Risk
>>>
>>> As I understand it, RR can only be used when the population prevalence
of
>>> the measured outcome is known. In many cases, population prevalence is
>>> not
>>> known, so OR can be properly used to approximate RR when quantifying
rare
>>> events. As events become more common, OR no longer accurately
>approximate
>>> RR. In these cases, OR will overestimate the benefits and harms of
>>> treatment (RR).
>>>
>>> See:
>>> Sackett DL. Down with odds ratios! Evidence-Based Medicine. 1996
>>> Sept/Oct;1:(got from ACP website so I don't have page numbers).
>>>
>>> -----Original Message-----
>>> From: Andrew Jull <[log in to unmask]>
>>> To: [log in to unmask]
>>> <[log in to unmask]>
>>> Date: Wednesday, February 24, 1999 11:41 AM
>>> Subject: Odds Ratios vs Relative Risk
>>>
>>>
>>> >Dear All
>>> >
>>> >I was recently conversing with a colleague and the question of why use
>>> ORs
>>> >instead RRs came up. My naive response was that the choice seemed to be
>>> >based on the individual's preference and that I had not read anything
>>> that
>>> >suggested the use of one was more informative than another (and indeed
>>> have
>>> >read some material that suggests ORs are misleading when the OR is
>high -
>>> >but I don't want to get onto that issue).
>>> >
>>> >Can anyone help me with why odds ratios might be used in preference to
>>> >relative risk or vice versa.
>>> >
>>> >regards
>>> >Andrew Jull
>>> >Clinical Nurse Consultant
>>> >Auckland Hospital
>>> >NEW ZEALAND
>>> >
>>
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