11

What are the formulaic parameter estimates for the skew-normal? If you can, the derivation via MLE or Mom would be great too. Thanks

Edit.

I have a set of data for which I can tell visually by plots is slightly skewed to the left. I want to estimate the mean and variance and then do a goodness-of-fit test (which is why I need the parameter estimates). Am I right in thinking I just have to guess the skew(alpha) (maybe do several skews and test for which is best?).

I would like the MLE derivation for my own understanding - would prefer MLE over MoM as I am more familiar with it.
I was unsure that there was more than one generic skew normal - I just mean a neg skewed mean! If possible, the skew exponential power param estimates would be helpful too!

Silverfish
  • 20,678
  • 23
  • 92
  • 180
user40124
  • 737
  • 10
  • 18
  • (1) which parameterization of which specific 'skew-normal'? (I've seen more than one thing get called that) (2) when you say "the formulaic parameter estimates" you imply (a) a closed form exists and (b) that there's only one --- yet you mention both ML and MoM, which won't generally be the same (& the ML estimators in particular might not be closed form). More information is required! – Glen_b Feb 17 '14 at 00:40
  • See, for instance, the paper by Vinod: _Skew Densities and Ensemble Inference for Financial Economics_, which illustrates how to fit data to a skew-Normal: http://mathematica-journal.com/issue/v9i4/SkewDensities.html – wolfies Apr 18 '14 at 07:04
  • 1
    In R, `snormFit` in `fGarch` will estimate a skew normal distribution, or you might prefer to look at the `sn` package (uses Azzalini's definition, beware that other definitions of "skew normal" exist). If you use Stata, try [here](http://www.stata-journal.com/article.html?article=st0207). [Various packages](http://azzalini.stat.unipd.it/SN/index.html) for Python, VBA and Perl are available from the site of Adelchi Azzalini at the University of Padua. – Silverfish Nov 26 '14 at 02:06

1 Answers1

7

Indeed, the "skew-normal family" has exploded in membership (the wikipedia article does not attest to this). So, let's consider the mother of them all, that has probability density function

$$f_X(x) = \frac{2}{\omega}\phi\left(\frac{x-\xi}{\omega}\right)\Phi\left(\alpha \left(\frac{x-\xi}{\omega}\right)\right)$$ where $\phi()$ is the standard normal pdf and $\Phi()$ the standard normal cdf. $\xi$ is the location parameter, $\omega$ is the scale parameter, and $\alpha$ is the skew parameter.

Closed-form solutions for the ML estimator do not exist. Method-of-Moments estimator provides closed forms as follows, assuming that all three parameters are non-zero (obviously if $\omega$ and/or $\xi$ are zero, then the steps below are simplified):

1) Obtain a MoM estimate $\hat \delta$ by solving for $\delta$ the expression for the skewness of the distribution,
enter image description here
using the estimated sample skewness coefficient $\hat \gamma_3$.

2) Obtain an estimate $\hat \alpha$ using

$$\delta = \frac {\alpha}{\sqrt {(1+\alpha^2)}} \implies \hat \alpha = \frac {\hat \delta}{\sqrt{1-\hat \delta^2}}$$

3) Obtain a MoM estimate $\hat \omega$ by solving for $\omega$ the expression for the variance, $$\hat \sigma^2_x = \omega^2\cdot \left(1-\frac{2\hat \delta^2}{\pi}\right)$$ using the sample variance and the estimated $\delta$ derived in the previous step

3) Obtain a MoM estimate $\hat \xi$ by solving for $\xi$ the expression for the mean of the distribution, $$\hat \mu_x = \xi + \hat \omega \hat \delta\sqrt {2/\pi}$$ using the sample mean and the previous estimates.

And don't forget to propagate estimation error in this sequential procedure, as regards the estimator variance.

Alecos Papadopoulos
  • 52,923
  • 5
  • 131
  • 241