Usually, we use a symmetric matrix for the `Sigma` parameter of `MASS::mvrnorm` function.

Usually, we use a symmetric matrix for the `Sigma` parameter of `MASS::mvrnorm` function.
```
# Fixed correlation
D_correlation = data.frame(MASS::mvrnorm(30, mu = c(0.9, 0.9), 
Sigma = matrix(c(1, 0.8, 1, 0.8), ncol = 2), empirical = TRUE))  # Correlated data
```

I think it should be `Sigma = matrix(c(1, 0.8, 0.8, 0.8), ncol = 2)`