No, I mentioned NOT subtracting.  You're not supposed to subtract!

Best, Susan
On Jul 12, 2007, at 4:10 PM, Rachel Myr wrote:

> Linda Palmer posted this URL, about the difference between relative  
> risk and
> odds ratio.  I found it lacked a clear explanation of exactly how  
> RR and OR
> are calculated using a four-celled grid, but parts of the  
> discussion were
> still edifying.  I'm including it again here.
>
> http://www.childrens-mercy.org/stats/journal/oddsratio.asp
>
> This article confirms what I thought, that odds ratios must be  
> inverted if
> you want to look at the odds the other way round.
> If you know the odds ratio for two things, and you want people to  
> compare
> them in the opposite roles, you invert the odds ratio.  You don't  
> subtract.
> Thus, if breastfed babies get an illness half as often as formula fed
> babies, you can say that the formula fed babies get the illness  
> twice as
> often as the breastfed babies.  The ratio one to two, inverted, is  
> two to
> one.  I am writing this in text because the characters I need to  
> set up the
> equation are the ones that typically get garbled on Lactnet.
>
> Susan mentioned subtracting, but I still can't figure out what I would
> subtract from what.
>
> Rachel Myr
> Kristiansand, Norway
>

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