def freeze_subjects(self):
        """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.
        """
        self.phenotype_data = numpy.array(self.phenotype_data)
        self.covariate_data = numpy.array(self.covariate_data)