docstr: This function calculates the negative predictive value ratio between
  the number of true negatives and the total number of negative elements
  
  :return:
docstr_tok: ["This","function","calculates","the","negative","predictive","value","ratio","between","the","number","of","true","negatives","and","the","total","number","of","negative","elements",":","return",":"]
code: def negative_predictive_values(self):
      """
      This function calculates the negative predictive value ratio between
      the number of true negatives and the total number of negative elements
  
      :return:
      """
      return self.tn() / (self.fn() + self.tn())
  
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 24 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.negative_predictive_values() and returns: unknown