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I just had a quick question that I hope someone can answer. Does anyone know what the distribution of the sum of discrete uniform random variables is? Is it a normal distribution?

Thanks!

Perdue
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  • First we want to specify that the discrete uniforms are independent and over the same interval. The sum is not normal, but the sum of a largish number of them is close enough to normal for most practical uses. – André Nicolas Apr 10 '13 at 18:45
  • So if I have a sample of n discrete uniforms, that would be close enough to normal for the purposes of finding a complete statistic? – Perdue Apr 10 '13 at 18:48
  • If you look up the defintion of complete statistic, you will find that the answer is no. In particular, for complete statistic the thing would have to woek for a sample of, say, $3$, where normal approximation is really not good. – André Nicolas Apr 10 '13 at 18:53
  • Crap...alright, I guess I am back to the drawing board on this one then. – Perdue Apr 10 '13 at 19:00
  • I don;t have time to give an answer, but for example if we are looking at the **continuous** uniform family in the interval $[0,\theta]$ where $\theta$ is a parameter, then the **maximum** of the $n$ observations is a complete statistic. This should be even more true for a similar family, discrete uniform on the interval $[0,\theta]$ where the parameter $\theta$ is an integer. – André Nicolas Apr 10 '13 at 19:28
  • The drawing board is always a good place to start. One wonders who upvoted this question? – wolfies Jun 23 '14 at 15:49

2 Answers2

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The distribution is asymptotically normal. Otherwise, the exact distribution is that of a normalized extended binomial coefficient, see "Polynomial Coefficients and Distribution of the Sum of Discrete Uniform Variables" by Camila C. S. Caiado and Pushpa N. Rathie at http://community.dur.ac.uk/c.c.d.s.caiado/multinomial.pdf.

Ludovic Kuty
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freder
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    This explanation seems to be more approachable: https://www.dartmouth.edu/~chance/teaching_aids/books_articles/probability_book/Chapter7.pdf – Casey Watson Feb 10 '17 at 04:24
  • For later reference (in case of 404 errors), the URL to the full book Introduction to Probability by Charles M. Grinstead and J. Laurie Snell is http://www.dartmouth.edu/~chance/teaching_aids/books_articles/probability_book/book.html – Ludovic Kuty May 12 '17 at 05:29
  • Hi Casey and Ludovic, Thanks for your information. I can find the paper written by Camila and Pushpa. But I still get confused about Polynomial Coefficient, expecially how to calculate it. May I ask how to calculate the Polynomial Coefficient by using software, say R or Matlab? – Bin Sep 29 '21 at 14:25
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So, the idea is to iteratively calculate PMF, using sum of $n-1$ variables to derive PMF of the sum of $n$ variables.

$$P_{a, b, n}(x) = \sum_{i \in a..b} P_{a, b, 1}(i) * P_{a, b, n-1}(x-i)$$

where $P_{a,b,n}$ is the PMF of the sum of $n$ discrete uniform variables.

Notional Python code to calculate the coefficients can be found here.

Marat
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