code: def mean_forecast_err(y, yhat):
      return y.sub(yhat).mean()
  
file_path: statsmodels/examples/python/tsa_arma_0.py
imports: import matplotlib.pyplot
  import numpy
  import pandas
  import statsmodels.api
  from scipy import stats
  from statsmodels.tsa.arima.model import ARIMA
  from statsmodels.graphics.api import qqplot
  from statsmodels.tsa.arima_process import ArmaProcess
label_desc: Use image node_id 1 for calling a global function with example usage: mean_forecast_err(y, yhat) and returns: y