I have a dichotomous DV and a single factor with three levels. Is it possible to test for a linear trend in the log-odds for each level of my factor?

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#### Best Answer

In R, you could specify that the factor is an *ordered* factor (`ordered`

), which will parameterize a three-level factor in terms of constant, linear, and quadratic terms … the `f.L`

parameter below essentially measures the linear trend.

`set.seed(101) d <- data.frame(y=rbinom(60,prob=0.5,size=1), f=ordered(rep(1:3,each=20))) summary(glm(y~f,data=d)) ## ... ## Coefficients: ## Estimate Std. Error t value Pr(>|t|) ## (Intercept) 0.51667 0.06612 7.815 1.4e-10 *** ## f.L -0.03536 0.11452 -0.309 0.759 ## f.Q 0.02041 0.11452 0.178 0.859 `

The `contr.sdif`

(successive differences) contrast from the `MASS`

package might also be of interest.

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