Function tangram_metrics::merge_mean_m2

pub fn merge_mean_m2(
    n_a: u64, 
    mean_a: f64, 
    m2_a: f64, 
    n_b: u64, 
    mean_b: f64, 
    m2_b: f64
) -> (f64, f64)

This function combines two separate means and variances into a single mean and variance which is useful in parallel algorithms.