# Negative variance with ratio and regression estimators

Considering multivariate stratified sampling for two study variables $y_1$,$y_2$ and two auxiliary variables $x_1$,$x_2$ with specific correlation, I generated populations for four strata with different parameters but this generated data set is giving me negative variance for ratio and regression estimators though I tried with different parameters. I need to ask what’s the possible reason for this? How should I set parameters to get positive variances for $y_1$ and $y_2$?