Calculates Xu' Omega-squared value, a simple R2 equivalent for linear mixed models.

## Details

`r2_xu()`

is a crude measure for the explained variance from
linear (mixed) effects models, which is originally denoted as
Ω^{2}.

## References

Xu, R. (2003). Measuring explained variation in linear mixed effects models. Statistics in Medicine, 22(22), 3527–3541. doi:10.1002/sim.1572

## Examples

```
model <- lm(Sepal.Length ~ Petal.Length + Species, data = iris)
r2_xu(model)
#> Xu's R2
#> 0.8367238
```