code: def get_bestnr(self, index=4.0, nhigh=3.0, null_snr_threshold=4.25,\
  		           null_grad_thresh=20., null_grad_val = 1./5.):
  		
  		# weight SNR by chisq
  		bestnr = self.get_new_snr(index=index, nhigh=nhigh,
  		                          column="chisq")
  		if len(self.get_ifos()) < 3:
  			return bestnr
  		# recontour null SNR threshold for higher SNRs
  		if self.snr > null_grad_thresh:
  			null_snr_threshold += (self.snr - null_grad_thresh) * null_grad_val
  		# weight SNR by null SNR
  		if self.get_null_snr() > null_snr_threshold:
  			bestnr /= 1 + self.get_null_snr() - null_snr_threshold
  		return bestnr
docs: Return the BestNR statistic for this row.