R help - Mar 2010 - variance of discrete uniform distribution

Hi all,

I am REALLY confused with the variance right now.

for a discrete uniform distribution on [1,12]

the mean is (1+12)/2=6.5
which is ok.

y=1:12
mean(y)

then var(y) 
gives me 13

1- on  http://en.wikipedia.org/wiki/Uniform_distribution_%28discrete%29 wiki 
    the variance is (12^2-1)/12=143/12

2- 
http://www.solvemymath.com/online_math_calculator/statistics/continuous_distributions/uniform/param_uniform.php
here 
    which used (12-1)^2/12=121/12

all different 3 answers!!!

All I am looking for is the variance of a random variable from discrete
uniform distribution.

Can someone clearify that for me please?

Thanks.

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On 9/03/2010, at 12:13 PM, casperyc wrote:
> 
> Hi all,
> 
> I am REALLY confused with the variance right now.
	You need to learn the difference

		(a) Between sample variance (*estimate* of population variance) and
		    population variance.

	and

		(b) Between discrete and continuous distributions.


	Given that you understand those differences you will see that all three
	answers are correct.

		cheers,

			Rolf Turner
> 
> for a discrete uniform distribution on [1,12]
> 
> the mean is (1+12)/2=6.5
> which is ok.
> 
> y=1:12
> mean(y)
> 
> then var(y) 
> gives me 13
> 
> 1- on  http://en.wikipedia.org/wiki/Uniform_distribution_%28discrete%29
wiki
>    the variance is (12^2-1)/12=143/12
> 
> 2- 
>
http://www.solvemymath.com/online_math_calculator/statistics/continuous_distributions/uniform/param_uniform.php
	***LOOK*** at the above.  Does it or does it not contain the string
	``continuous_distributions''???  And doesn't your question involve
	the ***discrete*** uniform distribution???

			R. T.
> here 
>    which used (12-1)^2/12=121/12
> 
> all different 3 answers!!!
> 
> All I am looking for is the variance of a random variable from discrete
> uniform distribution.
> 
> Can someone clearify that for me please?
> 
> Thanks.
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Hi Rolf Turner ,

God, it directed to the wrong page.

I firstly find the formula in wiki, than tried to verify the answer in R,
now, given that 143/12 ((n^2-1)/12 ) is the correct answer for a discrete
uniform random variable,
I am still not sure what R is calculating there?
why it gives me 13?

Thanks!
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