# Third and fourth central moment of a random variable

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## Summary:

In a paper published in the JOURNAL OF MATHEMATICAL PSYCHOLOGY 39, 265-274 (1995) a formula is given on page 272 for the expectation of a random variable (formula 23) and for it's variance (formula 24). Now I would like to know what the formulas look like for it's third and fourth central moment.

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The Poisson Inhibition Model

JAN C. dang AND AD H. G. S. VAN DER VEN

## Main Question or Discussion Point

My question is as follows. In the attached paper a formula is given on page 272 for the expectation of Tn (formula 23) and for the variance of Tn (formula 24). Now I would like to know what the formulas look like for Tn 's third and fourth central moment.

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mathman
General answer for any random variable $X$. Let $E(X)=\mu$ be the mean of $X$. Then the $n^{th}$ central moment is given by $E((X-\mu)^n)$.

Stephen Tashi
Now I would like to know what the formulas look like for Tn 's third and fourth central moment.
My guess is that you are asking for the third and fourth central moments of that particular $T_n$ rather than asking about the general concept of third and fourth moments. If that's the case, you are more likely to get an answer if you state the distribution of $T_n$ rather than hope that someone will read enough of the paper to figure that out.

Stephen Tashi The probability distribution is not given in the paper.

Stephen Tashi
Stephen Tashi The probability distribution is not given in the paper.
Is your question about how to infer the third and fourth moments of $T_n$ from what is given in the paper?

mjc123
Homework Helper
Tn (eq. 22) is a constant plus the difference between two variables with the same gamma distribution (eq. 15). If those two variables were independent it would be fairly simple, but they are correlated, and I'm not sure how to do that.

mjc123 Thanks a lot for your comment. I am going to look at equation (eq.15).

mjc123