R中具有参考水平的因素？

Question

In R there are factors and there are ordered factors. Is there anything "in between", so to speak? factors are completely unordered, while ordered factors are completely ordered; I want a factor type that has a reference level. This would be useful in tagging a control group, for instance, that I always want to consider as a "baseline".

I have seen relevel() in R, but this is not what I want, since there is no way (apparently) to tell the difference between a factor whose reference level is the first level, and a regular factor.

Does anyone know how to tag a reference level in R, or if not, how I can extend the factor class to do so?

Answer 1

You can customize the reference group using contrasts . For example, to set the contrast so that group2 is the reference in sleep , I would do this:

Without the contrast, group1 is the reference group.

> lm(extra~group, sleep)

Call:
lm(formula = extra ~ group, data = sleep)

Coefficients:
(Intercept)       group2  
       0.75         1.58  

With the contrast set:

> contrasts(sleep$group) <- contr.treatment(nlevels(sleep$group), base=2)
> lm(extra~group, sleep)

Call:
lm(formula = extra ~ group, data = sleep)

Coefficients:
(Intercept)       group1  
       2.33        -1.58