[R-sig-eco] Continuous (Non-Count) Skewed Data With Many Zeros

[Rich Shepard]

On Wed, 16 May 2012, Highland Statistics Ltd wrote:

> Non-normality of your response variable is not a reason to apply a data
> transformation.

Alain,

   I was exploring whether a transformation might be useful. Turns out that
it's not useful.

> It all depends, and no sensible answer can be given. 15% of zeros can
> screw things up....but it is also possible that 80% of zeros comply with a
> regression or GLM. For a discussion with examples see Chapter 10 in our
> 2012 book.

   I recognize there's no one correct answer. I'm seeking options because
I've not before had a reason to dig deeply into data like this. I've ordered
your 2012 book[1] and expect it to arrive Real Soon Now.

> Depends on the previous remark.....anything from linear regression to a
> zero inflated model for a continuous distributed response variable. There
> is just no simple answer possible. It all depends. But based on what you
> describe it will probably be something zero-inflated.

   Now I have a handfull of potential approaches to explore. I appreciate
your insights and comments as I delve into new areas.

Regards,

Rich

-- 
Richard B. Shepard, Ph.D.          |   Integrity - Credibility - Innovation
Applied Ecosystem Services, Inc.   |    Helping Ensure Our Clients' Futures
<http://www.appl-ecosys.com>     Voice: 503-667-4517      Fax: 503-667-8863

[1] I find your Analyzing Ecological Data and Mixed Effects Models and
Extentions in Ecology With R highly informative and useful. Thanks for
writing them.