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Having Trouble Understanding Chi Squared Stuff

  1. Nov 22, 2006 #1
    So I have my fit for my data (in this case it's the e/m lab, where the slope of my trend line = e/m).

    And I calculated the required y - (Ax + B) values, but I have no idea what to use for standard deviation to divide each value by. To be honest, I'm just lost in the notation. I remember in the lecture hearing that if all the STD's are the same, you can pull that out of the sum, but I have no idea which STD's he was talking about, since I have 3 different measured quantities.

    Also, degrees of freedom. Since I used 5 data points and had to measure 3 different quantities for each point, I would use D = 5 - 3 = 2?
  2. jcsd
  3. Nov 22, 2006 #2
    I figured it out! I just make up the data to make it look pretty! That's how you get all the grant money.
  4. Nov 23, 2006 #3
    No, seriously, I could still use some help.

    I think I figured out which STD to use. Since my y-values are averages from 5 sets of data to get 5 data points, I just use the STD for the 5 different sets of data as my 5 different STD's, right?

    I'm still not sure I understand the degree's of freedom, though.
  5. Nov 27, 2006 #4


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