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Finding upper and lower limits of a graph

  1. May 13, 2008 #1

    Aki

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    1. The problem statement, all variables and given/known data
    This is not really a homework problem, but it looks like a rather easy problem that I can't quite figure out.

    So I have a graph like shown in the attachment: a quantity vs time graph with some data points. I had no problem finding the average, but now I want to find both an upper and lower limits that would enclose 80% of the data points (so like what I have drawn in the graph). My question is: how can I do that using a simple program like Excel?


    2. Relevant equations
    I tried applying Chebycheff's Inequality
    http://en.wikipedia.org/wiki/Standard_deviation

    For 80% of the data points, I found the number of standard deviations from the average would be SquareRoot 5. So (SquareRoot 5 * Standard Deviation)/2 , and then add and subtract that from the average to find the limits.

    3. The attempt at a solution
    Chebycheff's Inequality worked for most graphs. However, for some graphs, I would get a negative value for the lower limit and that's not what I want to see.


    Thanks for the help!
     

    Attached Files:

    Last edited: May 13, 2008
  2. jcsd
  3. May 13, 2008 #2

    HallsofIvy

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    Staff Emeritus
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    Chebycheff's Inequality will give difference from the mean. Since here the mean is 80 I don't think you are in any danger of getting a negative lower limit!
     
  4. May 13, 2008 #3

    Aki

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    well I just made up that graph. I can't show the data that I'm working with because it's confidential. Anyhow, there are graphs where if I apply the Chebycheff's Inequality, I get a negative value for the lower limit.
     
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