///|
/// Calculates Standardized Mean Difference (SMD) for a single covariate between treatment and control groups.
pub fn smd(covariate : Array[Double], treatments : Array[Bool]) -> Double {
  let treated = Array::new()
  let control = Array::new()
  let n = if covariate.length() < treatments.length() {
    covariate.length()
  } else {
    treatments.length()
  }
  for i in 0..