I am attempting a portfolio optimization model and ended up generating negative portfolio variance using 2WaWbσaσbcorrel(a,b) or 2WaWb*Cov(a,b)
From reading the linked article where other users had an issue, I’m seeing that it is because the covariance matrix is not semi definite positive:
The solutions offered are for code, but I I need to use excel. Is there a way to generate a true covariance matrix within excel?
I’m also trying to wrap my head around what exactly semi-definite positive means and why what I’ve done won’t work. I understand that the portfolio variance cannot be negative. Within the linked post, another user states, “there exists no data set (with complete observations) from which you could have estimated such a covariance matrix”, but I don’t see why as covariance can be negative.