def get_mean_DEV(sum_ptrm_checks, sum_abs_ptrm_checks, n_pTRM, delta_x_prime):
    """
    input: sum_ptrm_checks, sum_abs_ptrm_checks, n_pTRM, delta_x_prime
    output: Mean deviation of a pTRM check
    """
    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