odds ratio: how to create reference
Philip Twumasi-Ankrah wrote:
To select another reference level from the default that R chooses you can use the "relevel" function
example;
Reorder Levels of Factorwarpbreaks$tension <- relevel(warpbreaks$tension, ref="M")
summary(lm(breaks ~ wool + tension, data=warpbreaks))
function findlink(pkg, fn) {
var Y, link;
Y = location.href.lastIndexOf("\\") + 1;
link = location.href.substring(0, Y);
link = link + "../../" + pkg + "/chtml/" + pkg + ".chm::/" + fn;
location.href = link;
}
This sets "M" as the reference level for the analysis. At the prompt on the R workspace, type
?relevel
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But Rewards Everything
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-Jane Porter
These calculations are automatic using the Design package, e.g. dd <- datadist(d); options(datadist='d') f <- lrm(y ~ pol(age,2)*sex+rcs(weight,4), data=d) summary(f, sex='m') # makes 'm' the reference cell if you don't like the default; gives odds ratios For the sex OR age is set to the median (default; easily changed; doesn't matter if interaction term omitted). Bunny in your original post it would be less confusing if you did not call levels of an independent variable the outcomes. Many people refer to the dependent variable as the outcome variable. Frank
--- On Tue, 9/23/08, Bunny, lautloscrew.com <bunny at lautloscrew.com> wrote: From: Bunny, lautloscrew.com <bunny at lautloscrew.com> Subject: [R] odds ratio: how to create reference To: r-help at r-project.org Date: Tuesday, September 23, 2008, 4:06 AM HI there, i know this is a basic question, though i need some help because this is somewhat away from my current issue, but nevertheless interesting to me... Lets assume i have some estimated probabilities, say estimated by a logit model. i know i can also state them as an odds ratio. Now i?d like to state these odds ratios as a reference to a specific outcome of my investigated variable. for example, if my covariate of interest is race and possible outcomes are white, black and hispanic, whereas the latter are minorities in my case - how can i state the odds ratio in such a way that white is the reference (always 1) and other races' odds ratio are relative to the reference. e.g. hispanics are 1.5 times more likely to ... Is creating 3 binary dummies for race the right way ? And if so how can i go on. As i said, i know this is rather basic, i am thankful for any links / references... thanks in advance !
Frank E Harrell Jr Professor and Chair School of Medicine
Department of Biostatistics Vanderbilt University