library(vcd)
library(car) # for Anova()
arth.polr <- polr(Improved ~ Sex + Treatment + Age, data = Arthritis)
summary(arth.polr)
## Re-fitting to get Hessian
## Call:
## polr(formula = Improved ~ Sex + Treatment + Age, data = Arthritis)
##
## Coefficients:
## Value Std. Error t value
## SexMale -1.2517 0.5464 -2.29
## TreatmentTreated 1.7453 0.4759 3.67
## Age 0.0382 0.0184 2.07
##
## Intercepts:
## Value Std. Error t value
## None|Some 2.532 1.057 2.395
## Some|Marked 3.431 1.091 3.144
##
## Residual Deviance: 145.46
## AIC: 155.46
Anova(arth.polr) # Type II tests
## Analysis of Deviance Table (Type II tests)
##
## Response: Improved
## LR Chisq Df Pr(>Chisq)
## Sex 5.69 1 0.01708 *
## Treatment 14.71 1 0.00013 ***
## Age 4.57 1 0.03251 *
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
library(effects)
arth.effects <- allEffects(arth.polr, xlevels = list(age = seq(15, 45, 5)))
plot(arth.effects) # visualize all main effects
