I have a situation where I have around 30 classes of variables with different means and variances (though the means aren't too far from eachother; think 4-7) and that the distributions are right skewed, and I am trying to do hypothesis testing on the sum of variables from these classes.

For example, sometimes I have 20 values sampled at random from these classes, and other times I have 50. When I plot the couple hundred sums I get a distribution that is close to normal.

Looking around I found the Lyapunov version of the central limit theorem. The only thing I'm catching myself on is the denominator in the normalization formula.

Normally I would take the standard deviation of the variables making up the sum and use that, but is that appropriate in this case? I believe it is but I'd like some confirmation or a source that goes over an application of Lyapunov to real data.

Edits based on comments:

Additional information: The distributions are positive right skewed

I've estimated the means of all the classes using a couple prior years of data, and can find the estimated standard deviations if needed.

What I'm trying to do is tell if a sum of n random variables that are a combination of different classes are statistically different from the sums of the estimated means of their classes.

So, suppose I have 20 values random chosen from the 30 classes (the same classes can be chosen more than once). I add up the values to a sum $X = x_1 + ... + x_{20}$. Is this significantly different from the sum of the means of the different classes represented here $\mu = \mu_1 + ... + \mu_{20}$?

  • $\begingroup$ do you know the means and variances of your classes? or can you at least estimate them? $\endgroup$
    – Aksakal
    Sep 9, 2021 at 15:29
  • 2
    $\begingroup$ There are a number of troubling aspects to this problem statement that need clarification. Is it really true you are lumping sums of 20 values in with sums of 50 values in your plot? Why? Second, no version of the CLT applies to data, because it's a statement about what eventually happens when a sample size becomes arbitrarily large. It would be more constructive for you to rewrite your question in terms of what you are really trying to do: tell us what the variables are, what your hypothesis is, and what test you are applying. $\endgroup$
    – whuber
    Sep 9, 2021 at 15:30
  • $\begingroup$ I added some additional comments. $\endgroup$ Sep 9, 2021 at 15:45
  • $\begingroup$ Are you perhaps asking how to obtain the distribution of the sum of $20$ randomly, independently chosen values from the $30$ classes? $\endgroup$
    – whuber
    Sep 9, 2021 at 16:32
  • $\begingroup$ @whuber more so I'm asking how I can check if the sum of 20 different values from the 30 classes is significant. $\endgroup$ Sep 9, 2021 at 16:37

1 Answer 1


if the variables are independent then you can apply Lyapunov's CLT. Simply look at the $\Delta=\sum_i x_i-\sum_i\mu_i$ and compare it to $s_n=\sqrt{ \sum_i\sigma_i^2}$, something like $\Delta>2s_n$ would be significant.

  • 1
    $\begingroup$ How, exactly, do you propose evaluating the Lyapunov condition? $\endgroup$
    – whuber
    Sep 9, 2021 at 16:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.