Ludovico Frate píše v St 29. 10. 2014 v 16:27 +0100: > Dear all,I'am trying to fit a very simple linear model. I am analyzing the > differences in the number of species (DS) found in several permanent plots in > two year of observations. > Firstly, I have calculated the differences per plot (i.e. number of species > in Plot 1 in Time A - number of species in Plot 1 in Time B and so on for all > the plots).Secondly, those differences were tested for deviation from zero by > means of a linear model > M2<-lm(DC~1, data = gransasso)summary(M2)E2<-residuals(M2)qqnorm(E2, pch = > 19, col = "blue"); qqline(E2, col = "red") > The qqnorm has shown that residuals were not normally distributed, thus I > need to use a GLM. However GLM (poisson family) does not work with negative > values (DS has negative values).I've tried to add a constant value to these > differences (i.e. +100) but the result is misleading since I am testing for > deviation from zero. > Do you have any suggestions? > Regards,Ludovico > > > > Ludovico > Frate > > PhD student (University of Molise - Italy) > Environmetrics Lab > http://www.distat.unimol.it/STAT/environmetrica/organico/collaboratori/ludovico-frate-1 > Department of Biosciences and Territory - DiBT > Universit del Molise. > Contrada Fonte > Lappone, > 86090 - Pesche (IS) > ITALIA. > Cel: ++39 > 3333767557 > Fax: ++39 (0874) 404123 > E-mail [email protected] > [email protected] > > [[alternative HTML version deleted]] > > _______________________________________________ > R-sig-ecology mailing list > [email protected] > https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
Hi Ludovico, correct me if I am wrong, but I think you want to say something like: "In X out of Y plots, number of species did substantially (statistically significantly?) changed". For this, I would separate the plots where number of species is bigger and those where opposite is true in the second census and perform binomial test: binom.test(x=abs(difference), n=more.species.i.e.bigger.of.the.numbers, p=0.5) per each plot. Maybe you need better p (something like mean probability of species to disappear,...) HTH. Martin -- ------------------------------ Pokud je tento e-mail součástí obchodního jednání, Přírodovědecká fakulta Univerzity Karlovy v Praze: a) si vyhrazuje právo jednání kdykoliv ukončit a to i bez uvedení důvodu, b) stanovuje, že smlouva musí mít písemnou formu, c) vylučuje přijetí nabídky s dodatkem či odchylkou, d) stanovuje, že smlouva je uzavřena teprve výslovným dosažením shody na všech náležitostech smlouvy. _______________________________________________ R-sig-ecology mailing list [email protected] https://stat.ethz.ch/mailman/listinfo/r-sig-ecology
