![Singular parameterizations, generalized inverses, and regression estimates](https://blogs.sas.com/content/iml/files/2018/11/ginv8.png)
I remember the first time I used PROC GLM in SAS to include a classification effect in a regression model. I thought I had done something wrong because the parameter estimates table was followed by a scary-looking note: Note: The X'X matrix has been found to be singular, and a