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Re: Precision of an experiment

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  • Re: Precision of an experiment


    Precision of a measuring device, by definition, is the repeatability you
    find using the device (like a tolerance on a milling machine for example).
    It is different from the accuracy of the device (how close the measurement
    is to the true value). To obtain the precision of something like a measuring
    device, I don' t know of any other method than collecting an adequate sample
    size on one standard test piece and calculate standard deviation, as you

    However, to me, when we talk about precision of an experiment, it is
    referring to the power (often ignored), defined as 1-beta, where:

    Beta = P(type II error)

    Thus, power can be interpreted as P(reject null hypothesis|hypothesis is
    Beta, however, is also dependent on sample size and standard deviation.

    The point: if your experiment is to determine difference within the samples,
    it sounds like a randomized complete block design will work well, using
    differing samples as blocks. If there are differences in samples, a post-hoc
    test can be used to determine which one(s) are significantly different

    Thus, the precision would be the magnitude of difference you can reject the
    null hypothesis with a specified power. This can be done via tables or with
    many statistical software packages.

    Any other thoughts on the topic out there?


    David Hebert
    Research Engineer, Motion Analysis Laboratory
    Shriners Hospital For Children
    Springfield, MA 01104-2396

    -----Original Message-----
    From: Barker, Dan (RGH) [mailtoan.Barker@RGH.SA.GOV.AU]
    Sent: Monday, March 27, 2000 6:04 AM
    Subject: Precision of an experiment

    Dear Biomech-l subscribers

    To calculate the precision of a measuring device or experiment it is usual
    to repeat the test a number of times ON ONE SPECIMEN, calculate the mean and
    SD of the variable of interest i.e. strain, and then calculate the
    coefficient of variation. CoV=standard dev./mean.

    Is there however another method to assess precision when (for example 4)
    DIFFERENT SAMPLES are tested twice each i.e. 8 measures (4 paired
    measurements)? Therefore instead of multiple repeated tests of one specimen,
    I have multiple specimens with only one repeated measure of each.

    Thanks in advance

    Dan Barker

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