Chi square test help needed

Hi everyone,

I have a confusion about the Chi square test as to what the
denominator in the formula means. I am trying to assess if my data
fits the Poisson distribution and I have the following set of values
for observed and expected values:

observed expected
6 7.44
5 4.97
9 5.47
2 5.15
13 11.34

Now the formula says that the chi-square is:

sum (observed - expected)^2 / (sigma)^2

Now, in case of Poisson distribution, what is this sigma? is it the
standard deviation or the error the measurement error.

So, is the sigma = standard deviation on observed data / sqrt(N) in
this case (N = 5).

Thanks for any help you can give me.

Luc
.

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