Basic questions on sampling sizes and basic question on sample means

Discussion in 'Probability' started by 2.7182818284590..., Oct 15, 2010.

  1. Hello Math-Heads!

    I work in a lab where we collect ~200 data points in a period. The
    data that we collect is blood sugar. OK, here is my VERY BASIC
    QUESTION:
    1. I am not sure how the prob. curve is shaped, but should we simply
    take a sample size of 30 for quality control processes?

    2. What are the governing equations describing all this?

    Finally, here is a very different question that I'm interested in:
    Suppose that the average height of men is 70" with a STDDEV of 2.5".

    Now, suppose that I collect 2 men per sample, and I measure their
    sizes. What would be the STDDEV of this statistical test? I realize
    that the STDDEV would be quite smaller, but what are the governing
    equations here?
     
    2.7182818284590..., Oct 15, 2010
    #1
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  2. 2.7182818284590...

    Jasen Betts Guest

    depends what you are going to do with them.
    I think you are looking for the chi-square test, or may atleast find
    the answer on this page:
    http://en.wikipedia.org/wiki/Chi-square_test#Chi-square_test_for_variance_in_a_normal_population
     
    Jasen Betts, Oct 17, 2010
    #2
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  3. 2.7182818284590...

    Peter Webb Guest

    ˙ʞɔıɹʇ Çɔıu
     
    Peter Webb, Oct 17, 2010
    #3

  4. Hello,

    In our blood sugar lab, we are going to take the mean and the STDDEV
    for quality control purposes. We are sure that if the average sugar
    level of patients is 100 with a small STDDEV, and one random week, the
    average is 125, we know that there is something wrong in the way we
    are taking data or there is a public health issue at hand. Most
    likely, however, there is something wrong with the way we are
    gathering data.

    My big question are these:
    1. Basically, how do we statistically sample all this? IE, do we use
    sample sizes of 30 every week? How many samples do we need to be
    within +/- 5% of the mean.

    For example, suppose that the average is 100, STDDEV is 10, and the
    total population is ~200. How many samples do we need to analyze for
    a margin of error to be 5%?

    2. If we can put all this data into Excel, what would be the
    DISADVANTAGES of simply analyzing ALL 200 weekly data points?
     
    2.7182818284590..., Oct 17, 2010
    #4
  5. 2.7182818284590...

    Jasen Betts Guest

    you need all of them to be certain to be withing 5% of the mean,

    it appears that you are not intending to re-test the samples but
    instead to just re-process the results, so why not put all of them
    into the computation?
    margin of error is an egineering term, in statistics the best you can
    do is a confidence interval, a confidence interval gets you a range
    and a probability.

    if the test says for examply you're 98% sure that the mean is between 97
    and 103 that's probably a good result but if it comes out 98% rpobability
    that it's between 95 and 99 you may want to investigate.

    the chi square test will tell you if the sample mean (mean of eg. 30
    measurements) is likely to be consistent with a population mean of 100.
    If you don't need to copy the data by hand I can see no disadvantages.

    If the data is in a database I would ask the database to give me
    the mean rather than copying the data and then processing it in excel.

    I was of the impression that laboratory tests were calibrated by using
    a standard solution of known concentration, why not confirm your
    processes that way?
     
    Jasen Betts, Oct 18, 2010
    #5



  6. Excellent answer my friend. I very much appreciate this.
     
    2.7182818284590..., Oct 20, 2010
    #6
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