class MoreMath::Sequence

def compute_welch_df(other)

degrees of freedom for Welch's t-test.
Use an approximation of the Welch-Satterthwaite equation to compute the 
def compute_welch_df(other)
  (sample_variance / size + other.sample_variance / other.size) ** 2 / (
    (sample_variance ** 2 / (size ** 2 * (size - 1))) +
    (other.sample_variance ** 2 / (other.size ** 2 * (other.size - 1))))
end