Dear R-helpers, I am using plm package to run panel data analysis for different models with different dependent variables. After running the PLM, I run the COEFTEST(plm1, vcovBK) to get the panel corrected standard errors. I got the PCSEs for some of the models but some tests produce error message. In one of the COEFTEST, the error message is: Error in solve.default(crossprod(demX)) : system is computationally singular: reciprocal condition number = 1.404e-017 What should I do to fix it and get the PCSEs? Thanks a lot, Hien Nguyen