class_docstr: Analyses for a collection of 2x2 contingency tables.
  
  Such a collection may arise by stratifying a single 2x2 table with
  respect to another factor.  This class implements the
  'Cochran-Mantel-Haenszel' and 'Breslow-Day' procedures for
  analyzing collections of 2x2 contingency tables.
  
  Parameters
  ----------
  tables : list or ndarray
      Either a list containing several 2x2 contingency tables, or
      a 2x2xk ndarray in which each slice along the third axis is a
      2x2 contingency table.
  
  Notes
  -----
  This results are based on a sampling model in which the units are
  independent both within and between strata.
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docstr: Estimate of the pooled risk ratio.
docstr_tok: ["Estimate","of","the","pooled","risk","ratio","."]
code: def riskratio_pooled(self):
      """
      Estimate of the pooled risk ratio.
      """
  
      acd = self.table[0, 0, :] * self._cpd
      cab = self.table[1, 0, :] * self._apb
  
      rr = np.sum(acd / self._n) / np.sum(cab / self._n)
      return rr
  
imports: import warnings
  import numpy
  import pandas
  from scipy import stats
  from statsmodels import iolib
  from statsmodels.tools import sm_exceptions
  from statsmodels.tools.decorators import cache_readonly
label_desc: Use image node_id 6 for calling the StratifiedTable obj's underlying member method code with example usage: obj.riskratio_pooled() and returns: rr