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Use of percentiles

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  • Use of percentiles

    Dear BIOMCH-l readers,

    Yesterday I posted a request concerning a formula to calculate
    percentiles. The replies were very helpfull although the main reply is:
    it cannot be done simply.

    That gets me to a more basic question:
    Why convert to percentiles at all?!

    When calculating z, by using the formula:

    z = (x-mu) / rho

    you get the number of standard deviations above or below the mean.
    Using the cumulative probability table you wille get the percentiles,
    which is, in my opinion the same value but only in another shape.

    So, why using percentiles (using tables or difficult calulation) while
    the z score already gives the answer.

    The only reason I can think of is that percentiles adress more to our
    imagination (we have some idea to what a percentile means, while the
    z-score remains more abstract)

    But maybe there are more legitimate reasons for using percentiles.
    Do you have any ideas on this topic?
    The main reason why I'm having these questions is that I'm comparing
    individual scores with populations and want to see at what level they
    score (strenght, mobility, endurance etc).

    By the way If anyone is interested in the replies I got please let me
    know. I dont think it will be a good idea to dump all the formula's on
    the discussion list right away.

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