code: def specificity(self):
      return self.tn() / self.n_neg_ref()
  
imports: from __future__ import absolute_import, print_function
  import numpy
  from scipy import ndimage
  from niftynet.utilities.util_common import MorphologyOps, CacheFunctionOutput
label_desc: Use image node_id 20 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.specificity() and returns: unknown