code: def positive_predictive_values(self):
      if self.flag_empty:
          return -1
      return self.tp() / (self.tp() + self.fp())
  
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 23 for calling the PairwiseMeasures obj's underlying member method code with example usage: obj.positive_predictive_values() and returns: unknown, unknown