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Statistic Problems With Quartiles and Standard Deviations

  1. Nov 5, 2007 #1
    There are two different problems that I am confused with:

    1) The taste of mature cheese is related to the concentration of lactic acid in cheese. Use the concentrations of lactic acid in 30 samples of cheddar cheese on page 15.

    Well, what's important is that using all the numbers it gave me, the mean is 1.44, the median is 1.45 and the standard deviation is .3035.

    Then it asks, "Calculate the percent of data that lie within one, two and three standard deviations. I attempted to work with one standard deviation, but I can't seem to get the right answer, which is 66% (or something close).

    What I did was... one standard deviation is 1.45 +/- .3035, which gets 1.15 and 1.75. Then you need to calculate the percentage of numbers that fall within that range.

    So, I take the z-score of each number: (1.15 - 1.45)/1.44 = -0.21 and it's the same for the other, except it comes out a positive 0.21 using 1.75 instead of 1.15. Using the A chart.. I get numbers of .4168 and .5832 respectively. Subtracting them gives me something like.. 12%. What am I doing wrong? Am I even in the right direction?

    The second question..is "How many standard deviations away from the mean do the quartiles lie in any normal distribution? What are the quartile for the lengths of human pregnancies? (which says that... for human pregnancies, the mean is 266 days and the Standard deviation is 16 days)

    Frankly, I have no idea how to start and I was hoping I would get a little push in the right direction.
     
  2. jcsd
  3. Nov 5, 2007 #2

    EnumaElish

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    z_ = 0 - 1 = -1 is the standard z score for "one std below" and z+ = 0 + 1 = +1 is for "one std above."
     
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