code: def get_mean_DEV(sum_ptrm_checks, sum_abs_ptrm_checks, n_pTRM, delta_x_prime):
      
      if not n_pTRM:
          return float('nan'), float('nan')
      mean_DEV = ((old_div(1., n_pTRM)) * (old_div(sum_ptrm_checks, delta_x_prime))) * 100
      mean_DEV_prime= ((old_div(1., n_pTRM)) * (old_div(sum_abs_ptrm_checks, delta_x_prime))) * 100
      return mean_DEV, mean_DEV_prime
docs: input: sum_ptrm_checks, sum_abs_ptrm_checks, n_pTRM, delta_x_prime
      output: Mean deviation of a pTRM check