So apologies if this sounds really basic, but I'm very new to meta-analysis, and this is my first ever time conducting one and using this software. I just wonder if someone could please guide me step-by-step in the most simplistic terms on how to actually include covariates? I would like to include age, gender, etc, but when I try to do this, I get the error 'gsl: lu.c:266: ERROR: matrix is singular gsl: lu.c:266: ERROR: matrix is singular Default GSL error handler invoked. gsl: lu.c:266: ERROR: matrix is singular gsl: lu.c:266: ERROR: matrix is singular gsl: lu.c:266: ERROR: matrix is singular' - All help is appreciated!