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# scikits.statsmodels.robust.norms.TukeyBiweight.psi

`TukeyBiweight.``psi`(z)

  The psi function for Tukey’s biweight estimator

  The analytic derivative of rho

  Parameters: | z : array-like 1d array
  Returns: | array : psi(z) = z*(1 - (z/c)**2)**2 for |z| <= R psi(z) = 0 for |z| > R

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