code: def freeze_subjects(self):
          
          self.phenotype_data = numpy.array(self.phenotype_data)
          self.covariate_data = numpy.array(self.covariate_data)
docs: Converts variable data into numpy arrays.
  
          This is required after all subjects have been added via the
          add_subject function, since we don't know ahead of time who is
          participating in the analysis due to various filtering possibilities.