Little help with weibull function needed

[kayard]

A simple (not for me) question !

how can I compute the mean and standard deviation of a weibull
distribution in excel ?

I need this because I'd like to find which alpha and beta gives a
weibull distribution with a specific mean and variance; so I was
thinking of using the solver and therefore I need a way to compute the
mean and variance given alpha and betas ...

Thanks in advance

Paolo

 

[Bondi]

Hi Paolo,

This link might help you,

http://www.qualitydigest.com/jan99/html/weibull.html

Regards,
Bondi

 

[Guest]

Bondi referenced a nice article for working with Weibull data, but the OP's
question of calculating moments from parameters is not answered there. For
that question, refer to
http://mathworld.wolfram.com/WeibullDistribution.html
In Excel, the mean of the Weibull(alpha,beta) distribution is given by
=beta*EXP(GAMMALN(1+1/alpha))
and the standard deviation is given by
=beta*SQRT(EXP(GAMMALN(1+2/alpha))-EXP(GAMMALN(1+1/alpha))^2)

Excel's GAMMALN is not very accurate for arguments very near 1 or 2 (<1
though that is irrelevant here), with the result that the standard deviation
calculation becomes increasingly inaccurate for alpha>500.

Jerry