Normal Plot

[sunil.chebolu@gmail.com]

I have a data set and I want to test if it is coming from a normal distribution. So I would like to use CODAP to get the Normal plot (aka QQ plot). Any suggestions on how I can do this?  Thank you for your help in advance.

[Bill Finzer]

Hello,

I’m sorry to say that CODAP does not have the ability to create the plot you want. If it had a function, let’s call it the “normalQuantile” function, we could do the following:

1. Compute the mean, standard deviation and count of the given data at the “parent” level of the dataset.
2. Sort the values in the dataset in ascending order.
3. Compute the percentile of each value using (caseIndex – 0.5) / count
4. Use the normalQuantile function to compute the value we would expect from a normally distributed random variable for each percentile.
5. Then we could plot the dataset values on the y-axis and the computed “normal quantile” values on the x-axis and we would have the desired plot.

But, as mentioned above, CODAP does not have the necessary normalQuantile function. But there’s no reason it couldn’t, so I’ll add that function as a desired feature.

Thanks for your interest in CODAP!

Bill

[Dan Damelin]

CODAP has the ability to generate a normal curve fit to your data.

To see this make a univariate plot of the values you are interested in. Then click on the measures tool (the ruler icon) to bring up the measures palette. Here you can click on “measures of spread” and click the box for “normal curve.” It would at least let your students see if their data visually fit a normal distribution.

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