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    Re: Instumental error?
    From: George Huxtable
    Date: 2005 Apr 20, 11:19 +0100

    Alex wrote-
    >I measure some distance several times, say 7 times in 10 minutes.
    >A star-to star distance can be considered constant for this period.
    >The average is the arithmetic average.
    >(x_1+x_2+x_3+x_4+x_5+x_6+x_7)/7 in this case.
    >Here x_1...x_7 are my sextant readings.
    >The standard deviation.
    >I subtract the average from each x_j, square the difference
    >add the results, divide by 6 and extract the
    >square root. (Actually the calculator does it all for me).
    One thing puzzles me. Why, in finding the mean of 7 squared-deviations from
    the average, does Alex (or his calculator) sum them and then divide by 6
    rather than by 7?
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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