how to find variance?

how to find the variance from these numbers: (it's poisson distribution)
20 24 24 22 23
21 20 22 23 22
21 21 22 21 23
22 20 22 20 24

I know a long way of doing it:

mean = 21.85

minus the mean from each number then square it
then divide it by 20

I got 0.07 as the answer.

Is there a quicker way?

You could knock 20 off each number to start with. It won't effect the variance, and the calculation becomes doable in your head - almost.

PS: I'd use E(X^2) - (E(X))^2 which avoids subtracting the mean from each numbers.

I tried that but for E(X^2) I got 103 (after dividing all the numbers by 20)

and the mean was 1.85

103 - 1.85^2 = 99.5775

is that right?

You get 103, as the sum of the squares after subtracting 20 from each number, but you still need to divide by 20.

So 103/20



Should be

103/20 - 1.85^2

would I keep the mean as the one after I divided each number by 20? or the original mean?