code: def npv(y, yhat, positive=True):
      """Returns the negative predictive value (higher is better).
  
      :param y: true function values
      :param yhat: predicted function values
      :param positive: the positive label
  
      :returns: number of true negative predictions / number of negative predictions
  
      """
      _, _, TN, FN = contingency_table(y, yhat, positive)
      return float(TN) / (TN + FN)
  
imports: import math
  import operator
label_desc: Use image node_id 17 for calling a global function with example usage: npv(y, yhat, positive) and returns: unknown