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Poisson variance derivation[edit]

Right now the Poisson section says:

The variance is equal to:
 \operatorname{Var}(X) = \sum_{k=1}^{n} \frac{\lambda^k}{k!} e^{-\lambda} (k-\lambda)^2 = \lambda,

It seems to me that this can't be right, since the value of the undefined n would affect the sum. Is n supposed to be infinity? Duoduoduo (talk) 14:03, 10 July 2013 (UTC)

True, and easy to check that the formula given above is false. Also the zero term is missing. Since all terms in the sum are non-negative, and the zero term is positive,
 \sum_{k=1}^n \frac{\lambda^k}{k!} e^{-\lambda}(k-\lambda)^2 <
\sum_{k=0}^\infty \frac{\lambda^k}{k!} e^{-\lambda} (k-\lambda)^2 = E(X-E[X])^2 = Var(X) = \lambda.

Mathstat (talk) 20:14, 10 July 2013 (UTC)

Formula for binomial variance in same section is also wrong. Mathstat (talk) 20:19, 10 July 2013 (UTC)

Meaning (interpretation) of Variance[edit]

While reading the article on variance, I found that a paragraph on how variance should be interpreted (in an intuitive way) was somehow missing. I don't feel confident to write that bit myself, but if someone could add this part it would I believe make the article more interesting and complete. — Preceding unsigned comment added by Marc saint ourens (talkcontribs) 18:29, 1 September 2013 (UTC)

Done. Thanks very much for the suggestion! Duoduoduo (talk) 16:56, 2 September 2013 (UTC)

Variance for six-sided die[edit]

Isn't the variance given in the article only correct for an infinite number of rolls of a die? For one roll, the variance is zero. For two rolls, a quick calculation suggests the variance is 5/8. Grover cleveland (talk) 16:27, 4 June 2014 (UTC)

Variance is not dependent on an expeiment, but only on properties of the die. You are confused with sample variance. Nijdam (talk) 20:06, 4 June 2014 (UTC)

Too encyclopedic and mathematical[edit]

normal people come here to look for simple applicable definition of variance and are overwhelmed by technical details.

I found this to be more usefull to me:

And I was distinguished Science graduate 20 years ago. — Preceding unsigned comment added by (talk) 03:06, 16 March 2015 (UTC)