code: def test_calculate_normal_reference_constant(self):
      const = self.constant
      kern = self.kern
      assert_allclose(const, kern.normal_reference_constant, 1e-2)
  
imports: import numpy
  from scipy import stats
  from statsmodels.sandbox.nonparametric import kernels
  from statsmodels.distributions.mixture_rvs import mixture_rvs
  from statsmodels.nonparametric.bandwidths import select_bandwidth
  from statsmodels.nonparametric.bandwidths import bw_normal_reference
  from numpy.testing import assert_allclose
  import pytest
label_desc: Use image node_id 1 for calling the CheckNormalReferenceConstant obj's underlying member method code with example usage: obj.test_calculate_normal_reference_constant() without return types