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Problem creating normal distribution curve

  1. #1
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    Angry Problem creating normal distribution curve

    Hi there...

    I want to share with you my problem.

    I have created an histogram with a set of values that have a standard deviation of 0.8547 and a Mean of 1.175.

    What I want to do is overlay a normal distribution curve over the histogram.

    I do know how to overlay, but I cannot manage to create the normal distribution curve.

    I tried the Normdist funcion and also tried generating random numbers using the mean and std dv values that I have.

    Nonetheless, when I generate those numbers and arrange them in my bins they do not follow a normal distribution (not even close!)... I cannot seem to see what the problem is.


    When I go to Data analysis > generate random numbers> number of variables =1 quantity of random numbers=40 Distribution=normal Mean= 1.175 and Standard dev=0.8547

    I get the following data:

    -0,691312356
    -0,635195811
    -0,403554738
    -0,26981251
    -0,203116299
    0,082964196
    0,246196955
    0,339420069
    0,513883521
    0,585082504
    0,615251268
    0,689594612
    0,727312369
    0,736361638
    0,829660543
    0,849082511
    0,858555433
    0,862613174
    0,895521108
    0,918391574
    0,974845291
    1,015891057
    1,102107376
    1,102763261
    1,290258905
    1,383766721
    1,635639172
    1,752040774
    1,82253043
    1,91489069
    1,94610304
    2,110915753
    2,199229932
    2,266001935
    2,322555736
    2,595046295
    2,656308864
    2,815097152
    2,860649576
    3,205472098

    When I calculate the frequency using the =Frequency funciton I get nothing near NORMAL...

    I also tried converting each of my values (the ones I used with the histogram) using the =normdist(x;mean;standard dev; true) formula... and I also do not get a normal dist.

    What can I do... this is killing me? Any other suggestion on how to overlay a normal curve to my histogram???

    THANK YOU!

  2. #2
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    Re: Problem creating normal distribution curve

    It might help to upload an example of your data, but I just knocked up a working example based on this:
    http://en.wikipedia.org/wiki/Normal_distribution
    I named two cells mean and variance, then had numbers from -6 to +6 in steps of 0.1 as my x column (starting in A5) and the function:
    =EXP(-((A5-mean)^2/2/variance^2))/SQRT(2*PI()*variance)
    in B5 copied down

    When plotted this made a very lovely bell curve, which I can manipulate by adjusting the mean and variance figures.

    hth

    PS, worth mentioning, variance = (standard deviation)^2
    Attached Files Attached Files
    Last edited by Cheeky Charlie; 10-20-2010 at 04:34 AM. Reason: add attachment, sd=sqrt(var)
    CC


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  3. #3
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    Re: Problem creating normal distribution curve

    Quote Originally Posted by Cheeky Charlie View Post
    It might help to upload an example of your data, but I just knocked up a working example based on this:
    http://en.wikipedia.org/wiki/Normal_distribution
    I named two cells mean and variance, then had numbers from -6 to +6 in steps of 0.1 as my x column (starting in A5) and the function:
    =EXP(-((A5-mean)^2/2/variance^2))/SQRT(2*PI()*variance)
    in B5 copied down

    When plotted this made a very lovely bell curve, which I can manipulate by adjusting the mean and variance figures.

    hth

    PS, worth mentioning, variance = (standard deviation)^2

    Hey It worked with your attachment! Thank you very much!!
    Last edited by goodmountain; 10-20-2010 at 01:49 PM. Reason: added the PS.

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