code: def first_order(A, AB, B):
      """
      First order estimator following Saltelli et al. 2010 CPC, normalized by
      sample variance
      """
      y = np.r_[A, B]
      if y.ptp() == 0:
          warn(CONST_RESULT_MSG)
          return np.array([0.0])
  
      return np.mean(B * (AB - A), axis=0) / np.var(y, axis=0)
  
imports: from types import MethodType
  from warnings import warn
  from scipy.stats import norm
  import numpy
  import pandas
  from .None import common_args
  from ..util import read_param_file, ResultDict, extract_group_names, _check_groups
  from multiprocessing import Pool, cpu_count
  from functools import partial
  from itertools import combinations, zip_longest
label_desc: Use image node_id 2 for calling a global function with example usage: first_order(A, AB, B) and returns: unknown, np