1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Finding upper and lower limits of a graph

  1. May 13, 2008 #1


    User Avatar

    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" [Broken]

    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 by a moderator: May 3, 2017
  2. jcsd
  3. May 13, 2008 #2


    User Avatar
    Science Advisor

    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


    User Avatar

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook