sas 默认的应该是effect coding
具体可以看下面两条链接
http://www.ats.ucla.edu/stat/mult_pkg/faq/general/effect.htm
http://www.ats.ucla.edu/stat/mult_pkg/faq/general/dummy.htm

我猜一般心里有一个所谓的参照组的时候，可以用reference coding，将那个参照组设置为0

why use effect coding instead of dummy coding? If you have several categorical variables in a model it often doesn't make much difference whether you use effect coding or dummy coding. However, if you have an interaction of two categorical variables then effect coding may provide some benefits. The primary benefit is that you get reasonable estimates of both the main effects and interaction using effect coding. With dummy coding the estimate of the interaction is fine but main effects are not "true" main effects but rather what are called simple effects, i.e., the effect of one variable at one level of the other variable. This is why most analysis of variance programs use some type of effect coding when estimating the various effects in an ANOVA model.

谢谢大神！大概对效应和引用的差别有点理解了，能不能简单解答下第二个问题