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    Re: Multi-Moon line exercise in 2 parts
    From: Peter Hakel
    Date: 2009 Aug 8, 09:29 -0700
    I find it interesting that a single outlier among 30 data points can have such a significant effect on the final result.  Clearly, a good software program expected to function as a "black box" (i.e. without needing human intervention to filter out bad data points on input) must carry out the statistics very carefully - and, presumably, display a warning message.

    Peter Hakel

    From: George Huxtable <george@hux.me.uk>
    To: NavList@fer3.com
    Sent: Saturday, August 8, 2009 6:36:14 AM
    Subject: [NavList 9412] Re: Multi-Moon line exercise in 2 parts

    Peter Hakel (PH) wrote, in [9409], about Jeremy's closely-spaced sequences
    of Moon altitudes, in [9359]-

    "An important detail is that I obtained this result by disregarding the
    second measurement (at 8:54:46) because its intercept does not seem to
    "belong" among all the others (it is AWAY, while all others are TOWARD).  If
    I do include this outlier, the results get worse:"

    Yes. A simple plot of Jermy's data "Moonlines, Moon away from LAM" shows
    that the observation at 8.54.56 is a real misfit, and should be discarded.
    It seems most likely that there was a recording or transcribing error, and
    that it should have been noted as 8.52.56, but it's not safe to guess at
    such things, and the only safe procedure is to delete that line. Peter did
    the right thing.

    What, by the way, do the letters LAM stand for?

    With that line deleted, those two data sets show remarkably little scatter.
    A least-squares fit to the non-culminating series arrives at a rms scatter
    of little more that 0.1 arc-minute, and of the series around culmination of
    about 0.15 arc-min. These show only random errors, not systematic ones such
    as those due to index error observation or dip; they show what can be
    achieved by a careful observer on a large vessel in, presumably, benign


    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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