class MoreMath::Sequence

def durbin_watson_statistic

for positive, d >> 2 for negative and d around 2 for no autocorrelation.
Returns the d-value for the Durbin-Watson statistic. The value is d << 2 
def durbin_watson_statistic
  e = linear_regression.residuals
  e.size <= 1 and return 2.0
  (1...e.size).inject(0.0) { |s, i| s + (e[i] - e[i - 1]) ** 2 } /
    e.inject(0.0) { |s, x| s + x ** 2 }
end