Ernesto There are several ways of tackling skewed data with zeroes and I am sure you will get emails from proponents of this or that other contributor.
Ways which I have found useful: (1) try a lognormal probability plot and see whether you have a straight line or if it drops off the line at low values. This is indicative of a three parameter lognormal distribution which needs an additive constant. Find the additive constant that makes the line straightest (my criterion) or the skewness closest to zero (Sichel's recommendation). You can find this described in my 1987 paper following Sichel's definitive works. Full copy at http://uk.geocities.com/drisobelclark/resume/Publications.html {paper titled "turning the tables...." (2) treat the zeroes as a different population. Are they zero because there are no fish there or because you didn't catch any? If the later, use an indicator approach to separate the 'no fish' population from the 'some fish' one. Then do your lognormal stuff on the 'some fish' and recombine for final results. (3) - not so nice: use the probability plot as suggested above to choose a 'threshhold' value to replace the zeroes. This assumes that all areas sampled are 'some fish' areas and you just didn't catch any. Isobel Clark http://uk.geocities.com/geoecosse/news.html __________________________________________________ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and "unsubscribe ai-geostats" followed by "end" on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org
