Sum and Difference of Poisson Distributions

For 2 independent Poisson distributions, X~Po(ʎ) and Y~Po(µ)

X+Y ~Po(ʎ+µ)

X-Y does NOT follow Poisson distribution.

If X and Y fulfill the conditions for using Normal approximation to Poisson, then X and Y can be approximated individually using Normal approximation.

For example X~ Po(30) and Y~ Po(20)

X~N(30,30) approximately and Y ~N(20,20) approximately

Then X-Y~N(10,50) approximately. Continuity correction applies.

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