[R] Percentage cover data with many zeros

[David Winsemius]

On Jan 24, 2015, at 8:37 AM, peter dalgaard wrote:

> Don't worry, there are plenty of halfwits around here. However, this is about stats theory,  and not really about R, so you're better off trying CrossValidated, aka stats.stackexchange.com

This is useful and correct advice, but if you want to see an excellent description of some of the R tools for dealing with zero-inflation, the Zeileis, Kleiber & Jackman article on the matter: http://www.jstatsoft.org/v27/i08/paper ,  was very helpful in advancing my understanding of the subject area.

> 
> -pd
> 
>> On 24 Jan 2015, at 14:26 , Ben Brooker <awe.ben at googlemail.com> wrote:
>> 
>> Hi,
>> 
>> I am new to R and have not had the most exposure to statistics.
>> I have a dataset of percentage cover (so 0-100) for certain species in 3
>> different shore zones (High, mid and low). The data was recorded for
>> different protected areas as well (17 of them) and my number of obs is
>> large (3358). I'm obviously interested in the difference in percentage
>> cover of species between shore zones as well as between protected areas.
>> The problem is that my data contains loads of zeros and I haven't dealt yet
>> in statistics with how to manipulate the data so as to perform robust tests
>> on it. I previously used Kruskal-Wallis ANOVAs

I wonder if the terms Kruskal-Wallis and ANOVA should be adjacent. I do not remember that variances are part of the inference with KW-tests. You might ask that in your question to the very helpful group on CrossValidated.com


-- 
David.

>> to look at cover differences
>> in shore zone but I am worried that it is inappropriate because of the
>> large sample size that I have and because my variances are not equal.
>> 
>> I've read a bit about using a zero-inflated negative binomial regression to
>> fit to my data, but I'm not sure if that will work because it is for count
>> data.
>> 
>> I would very much appreciate it if someone could point me in the correct
>> direction wrt a transformation that may help or an appropriate model to fit
>> or test to use. I've searched quite a bit but I'm a out of my depth.
>> 
>> PS sorry if I sound like a halfwit
>> 
>> Thanks a lot
>> 
>> Ben
>>