code: def _get_comparators(self, mf: MetricFrame, aggregation: str):
      if aggregation == "min":
          actual = mf.group_min_ci()
          nominal = mf.group_min()
      elif aggregation == "max":
          actual = mf.group_max_ci()
          nominal = mf.group_max()
      else:
          raise ValueError(f"Unrecognised option: {aggregation}")
  
      return actual, nominal
  
imports: import numpy
  import pytest
  import sklearn.metrics
  from fairlearn.metrics import MetricFrame, count
  from .data_for_test import g_1, g_2, y_p, y_t
label_desc: Use image node_id 1 for calling the TestGroupExtremes obj's underlying member method code with example usage: obj._get_comparators(mf, aggregation) and returns: actual,  nominal