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    Re: Rejecting outliers
    From: George Huxtable
    Date: 2011 Jan 8, 14:10 -0000

    I agree with (almost) every word from Gary LaPook in his thoughtful
    He sets two standard deviations from the mean as the maximum acceptable
    before excluding an observation as an outlier in a set of five or six.
    Personally, I would set that threshold somewhat higher, to say 2.5 SD. And
    his data-set is so near to a straight-line that graphing a straight-line,
    then measuring offsets with dividers, though fine in principle, is somewhat
    crude, and I would prefer to calculate those offsets numerically. But
    really, there's little to separate us on those matters.
    But I think that there's a serious weakness in his adoption of a personal,
    historical, value for standard deviation of his measured altitudes of
    1.433', and using twice that value to set an acceptability threshold for a
    particlar day. That may indeed have been the standard devition of his
    altitude observations from GPS values over a long term, but those will not
    all have been measured under the same circumstances. Some may have been
    taken from smaller vessels in fair weather, others in rough conditions.
    Some may even have been taken on land. It may be fair to lump together the
    rough with the smooth, and arrive at a personal average, but then what
    needs to be considered are the circumstances of the moment, and how they
    differ from that average.
    So if most had been taken from the deck of a craft of a few (or a few tens
    of) tons, how should that apply to observations from a vessel of many
    thousands of tons, with or without sail propulsion? If some of them had
    been taken in stormy conditions, even aboard Royal Clipper, much greater
    scatter would be expected, and no doubt many, perhaps most, of the
    resulting observations would fail Gary's test and be excluded, even though
    they were not really outliers from the expected (wider)distribution.
    To which, Gary may well ask what value he should use instead. And I suggest
    his best measure of scatter is the standard deviation of his observations
    of that day, about their mean value, having corrected them for
    time-dependence. The trouble is that to get a decent figure for that
    scatter calls for a reasonable number of observations, to avoid the result
    being overdependent on chance. Six observations isn't really enough. Nine,
    as in Peter Fogg's example, is still somewhat short. The more, the better.
    But Gary's personal experience of his ability to measure altitudes, from
    the past, can not be dismissed, but used to inform the choice of a suitable
    value for the scatter pertaining to that day's conditions. There may,
    indeed, still be a bit of 'magic' left in the navigator's art, in the
    making of such decisions.
    contact George Huxtable, at george{at}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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