def METHOD_NAME(p, fcn, data, bg, *args):
    """Helper function which evaluates a model function (fcn) with parameters (p) and additional arguments
    (*args) and compares this with measured data (data)"""
    f0 = poisson_lhood(p, fcn, data, bg, *args)
    df = 0*p
    for i in range(len(p)):
        dpi = 0.1*p[i] + 1
        pt = 1.0*p + 0
        pt[i] += dpi
        ft = poisson_lhood(pt, fcn, data, bg, *args)
        df[i] = (ft - f0)/dpi
    return df