At 16:19 11/03/2007, John Gerlach wrote: >After lengthly reviews of the literature drilling down through the >numerous citations that all cite secondary sources, I found that all >of the statements that ANOVA is robust to normality or homogeneity >were based on a couple of early simulations using one-way models. >All bets are off once you leave the simplistic realm of one-way ANOVA.
Similar statements are made within linear regression (and anova is linear regression)..... Montgomery and Peck (2002?). >In my analysis the distributions of normality or homogeneity >patterns across the data structure were critically important for >interpreting effects. After a lot of pain, including failing to get >proc GLM to run without crashing, If a GLM fails (in whichever package) I would rather try to understand why it fails. To me, that is more a warning that something "funny" goes on with your data. Perhaps a certain combination of factors with not enough observations? Alain >I went with a weighted ANOVA approach for Case 2 and for Case 1 I'll >probably use a detection limit approach that is used to analyze >water quality data - failure time approaches don't lend themselves >to factorial ANOVA. > >John Gerlach > > > >----- Original Message ---- >From: Highland Statistics Ltd. <[EMAIL PROTECTED]> >To: ECOLOG-L@LISTSERV.UMD.EDU >Sent: Sunday, March 11, 2007 4:38:10 AM >Subject: Re: [ECOLOG-L] Dealing with non-normal, ordinal data for >2-way ANOVA with interactions > >On Wed, 7 Mar 2007 16:19:31 -0500, Ryan Earley <[EMAIL PROTECTED]> >wrote: > > >Help with stubbornly non-normal data.... > > > >We have a data set with 2 independent variables and 1 dependent (Gosner > >stage for amphibian larvae). > >Hello, >Normality is less important. What about homogeneity? > >We have tried every creative way to transform > >the data > >a waste of your time I am afraid > > >and end up with significant deviation from normality each time. > >Just make a histogram or QQ plot, and judge by eye. Normality is not soo >important....compared to independence and homogeneity. But it also depends >on sample size, whether the data are balanced and how significant your >results are. And perhaps your non-normality is caused by an improper model? >See also: ><http://www.springer.com/0-387-45967-7>www.springer.com/0-387-45967-7 >for possible solutions. > > >What we'd like to ultimately do is test both main effects and their > >testing the main efffects while the interaction is significant??? There is >a whole discussion on this topic. See Underwood (200-something). > > >interaction (which effectively eliminates the use of two Kruskal-Wallis > >tests or Friedman's two-way ANOVA). We would be indebted to anyone who >might > >Is your response (dependent) ordinal??? Then I guess it has only a few >unqiue values....? No wonder it is not normal. In thas case, have a look >at multinomial logistic regression (MLR). There is also an "extension" of >MLR that takes into account the fact that the data are ordinal. See: > >Kleinbaum DG Klein M (2002) Logistic Regression A Self-Learning Text. New >York: Springer-Verlag > > >Alain > >Dr. Alain F. Zuur >First author of: > >Analysing Ecological Data (2007). Zuur, AF, Ieno, EN and Smith, GM. >Springer. 680 p. >URL: <http://www.springer.com/0-387-45967-7>www.springer.com/0-387-45967-7 > >Analysing Ecological data using GLMM and GAMM in R. (2008). Zuur, AF, >Ieno, EN, Walker, N and Smith, GM >Springer. > >Other books: ><http://www.brodgar.com/books.htm>http://www.brodgar.com/books.htm > >Statistical consultancy, courses, data analysis and software >Highland Statistics Ltd. >6 Laverock road >UK - AB41 6FN Newburgh >Tel: 0044 1358 788177 >Email: [EMAIL PROTECTED] >URL: <http://www.highstat.com>www.highstat.com >URL: <http://www.brodgar.com>www.brodgar.com > > > > > > > > > >have a suggestion on how to proceed statistically. Thanks for your help >in > >advance. > > > > > > > >Best, > >Ryan L. Earley & Foung Vang > >Cal State Fresno > >========================================================================= > >No virus found in this incoming message. >Checked by AVG Free Edition. >Version: 7.5.446 / Virus Database: 268.18.8/718 - Release Date: >11/03/2007 09:27 Dr. Alain F. Zuur First author of: Analysing Ecological Data (2007). Zuur, AF, Ieno, EN and Smith, GM. Springer. 680 p. URL: www.springer.com/0-387-45967-7 Analysing Ecological data using GLMM and GAMM in R. (2008). Zuur, AF, Ieno, EN, Walker, N and Smith, GM Springer. Other books: http://www.brodgar.com/books.htm Statistical consultancy, courses, data analysis and software Highland Statistics Ltd. 6 Laverock road UK - AB41 6FN Newburgh Tel: 0044 1358 788177 Email: [EMAIL PROTECTED] URL: www.highstat.com URL: www.brodgar.com
