def METHOD_NAME(self, testdata):
    """
    Tests using different window overlap percentages.
    Regression test for bug #1821.
    """
    st1 = read(testdata['ref_STS2'])
    st2 = read(testdata['ref_unknown'])
    calfile = testdata['STS2_simp.cal']
    def median_amplitude_plateau(freq, amp):
        # resulting response is pretty much flat in this frequency range
        return np.median(amp[(freq >= 0.3) & (freq <= 3)])
    # correct results using default overlap fraction of 0.5
    freq, amp, phase = rel_calib_stack(
        st1, st2, calfile, 20, smooth=10, overlap_frac=0.5,
        save_data=False)
    amp_expected = median_amplitude_plateau(freq, amp)
    for overlap in np.linspace(0.1, 0.9, 5):
        freq2, amp2, phase2 = rel_calib_stack(
            st1, st2, calfile, 20, smooth=10, overlap_frac=overlap,
            save_data=False)
        amp_got = median_amplitude_plateau(freq2, amp2)
        percentual_difference = abs(
            (amp_expected - amp_got) / amp_expected)
        # make sure results are close for any overlap choice
        assert percentual_difference < 0.01