code: def test_no_NDs(self):
      _df = self.df.copy()
      _df["qual"] = False
      result = ros.cohn_numbers(
          _df, observations="conc", censorship="qual"
      )
      assert result.shape == (0, 6)
  
imports: from statsmodels.compat.pandas import assert_series_equal, assert_frame_equal
  from io import StringIO
  from textwrap import dedent
  import numpy
  import numpy.testing
  import numpy
  from numpy.testing import assert_equal
  import pandas
  import pytest
  from statsmodels.imputation import ros
label_desc: Use image node_id 3 for calling the Test_cohn_numbers obj's underlying member method code with example usage: obj.test_no_NDs() without return types