Thank you very much for your reply. The very low colonization rates for one of the treatments are observed in all four blocks. I guess therefore that I can exlude this treatment and test the relative effects of the three remaining treatments in a 3-by-4 ANOVA followed by a multiple comparison test.
L�naick Donald Burrill a �crit : > > You really don't want to include a "zero mean zero variance" treatment > in your analysis: it will generate a main effect of treatment, and the > "zero variance" part will cause your error mean square to be > underestimated, leading to what one might call spurious sensitivity to > treatment effects. And it isn't as if you needed to find out from the > analysis that this treatment differs from the others -- you know that > already. > > For the treatment that was "poorly colonized", is that true of all four > blocks? If so, I'd recommend eliminating that treatment from the > analysis. You have essentially no useful information about it, in terms > of the variable you're analyzing. (Except, perhaps, that the treatment > is lethal.) If the treatment really does have, as you write, zero mean > and zero variance, you don't need a formal ANOVA to tell you that it > differs from the other treatments. In a 3-by-4 ANOVA you can then > assess the relative effects of the other treatments. > > If, instead, the "zero mean zero variance" phenomenon applies only to > one of the 16 block-by-treatment conditions in your experiment, there is > a way of adjusting for that particular condition while pursuing effects > in the rest of the experimental design. But this is not a standard form > of analysis, and will take some explaining. I'll wait for clarification > before trying to describe it. > > On Thu, 15 Apr 2004, Lenaick MENOT wrote: > > > I have conducted a colonization experiment invloving four treatments > > each with four replicates, the replicates were randomly placed in four > > blocks. I would like to use a two way ANOVA without replication to > > test the effects of treatments but one of the treatment was poorly > > colonized and most of the taxa are lacking. > > > > My question is could I perform an ANOVA including the treatment with > > zero mean, zero variance, should I exclude the treatment with zero > > mean zero variance from the analysis or is there any special procedure > > to deal with zero mean zero variance in an analysis of variance? > > > > Thank you for your help. > > > > L�naick Menot > > ------------------------------------------------------------ > Donald F. Burrill [EMAIL PROTECTED] > 56 Sebbins Pond Drive, Bedford, NH 03110 (603) 626-0816 > . > . > ================================================================= > Instructions for joining and leaving this list, remarks about the > problem of INAPPROPRIATE MESSAGES, and archives are available at: > . http://jse.stat.ncsu.edu/ . > ================================================================= -- __________________________________________ DRO/Departement Environnement Profond Ifremer, Centre de Brest BP 70 29280 Plouzane FRANCE Tel.: (33) 2 98 22 45 52 Tel. Secr�tariat : (33) 2 98 22 43 04 Fax : (33) 2 98 22 47 57 http://www.ifremer.fr/droep/ Cedre Tel.: (33) 2 98 33 10 10 ___________________________________________ . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
