From: Mark Strovink (strovink@lbl.gov)
Date: Tue Sep 07 2004 - 07:55:16 PDT
Hi Alex,
I found your 8/25 draft to be interesting, educational, and
intelligently
written.
My comments concern [the data in] Figs. 9, 10 and 12.
----------
In Fig. 9, errors corresponding to a peculiar velocity of 300 km/sec are
displayed horizontally. In quadrature, scaled by the local Hubble
slope,
are they also included in the vertical errors? If not, perhaps this
should
be considered; it would help to dispel the false impression that the
lowest-z points contribute disproportionately to chi^2.
----------
From Fig. 10, one hopes to judge whether the stretch-luminosity
relationship
is systematically different for high-z and low-z SNe. However, since
the
chi^2 of the low-z points with respect to the solid line is obviously
unacceptable, one is led not to take seriously their displayed error
bars.
In turn this confounds the judgment.
Is this conundrum avoidable? Perhaps the following supplement to Fig.
10
should be considered for inclusion: Start from the residuals and their
errors (relative to "Best Fit") from Fig. 9 (see above comment). Modify
these residuals by backing out the stretch correction to each point (but
retain the stretch-correction-related contribution to the error in the
plotted residual). Using these residuals and their errors, construct
the
analog to Fig. 10.
----------
In Fig. 12, no errors are shown. Based on the points displayed there, a
Pearson coefficient and two best-fit (slopes + errors) are quoted. Do
the
Pearson coefficient and best-fit slopes take into account the known
point-by-point errors, both in B_BV residual and B_max residual, that
could
have been displayed in Fig. 12?
----------
Assuming that no qualitatively different impressions result from these
modifications, none of the above comments imply that the proposed cuts
should not now be finalized and the data should not now be unblinded.
- Mark
[Strovink@lbl.gov, LBL (510)486-7087 (Fax 4047), FNAL (630)840-8715]
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