code: def EI(mean, std, max_val, tradeoff):
      z = (mean - max_val - tradeoff) / std
      return (mean - max_val - tradeoff) * ndtr(z) + std * norm.pdf(z)
  
file_path: modAL/modAL/acquisition.py
imports: import numpy
  from scipy.special import ndtr
  from scipy.stats import norm
  from sklearn.exceptions import NotFittedError
  from modAL.models.base import BaseLearner
  from modAL.utils.data import modALinput
  from modAL.utils.selection import multi_argmax
label_desc: Use image node_id 2 for calling a global function with example usage: EI(mean, std, max_val, tradeoff) and returns: unknown