This isn't a SAS question, it's a statistics question. SAS is simply a program for applying statistical knowledge, you still need to understand the statistical methodology.   If you have a variable that can be expressed as a linear combination of other variables, it is not independent. So if you have 0/1 for every day, then you have overspecified the model and need to remove one of the day variables.    A more detailed write up: http://stats.idre.ucla.edu/other/mult-pkg/faq/general/faqwhat-is-dummy-coding/     ... View more