Eric
I downloaded Dave Sackett's article from EBM which made some sense, but your
clarification has actually done the trick. Thank you.
regards
Andrew
> -----Original Message-----
> From: Eric Harvey [SMTP:[log in to unmask]]
> Sent: Friday, 26 February 1999 05:01
> To: [log in to unmask]
> 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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