Dear Tobias, Your observation that "When plot [the residuals from?] this model I get a banana-shape in Normal Q-Q Plot(with open site [side?] pointing downwards)," suggests that the residuals are negatively skewed, which in turn suggests that using log(wt) as the response variable may have been ill-advised. Perhaps simply using wt, or a weaker transformation such as sqrt(wt), would produce better-behaved residuals.
I hope this helps, John ------------------------------ John Fox, Professor Department of Sociology McMaster University Hamilton, Ontario, Canada web: socserv.mcmaster.ca/jfox > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On > Behalf Of Tobias Erik Reiners > Sent: May-04-08 5:56 AM > To: r-help@r-project.org > Subject: [R] Ancova_non-normality of errors > > Hello Helpers, > > I have some problems with fitting the model for my data... > -->my Literatur says (crawley testbook)= > Non-normality of errors-->I get a banana shape Q-Q plot with opening > of banana downwards > > Structure of data: > origin wt pes gender > 1 wild 5.35 147.0 male > 2 wild 5.90 148.0 male > 3 wild 6.00 156.0 male > 4 wild 7.50 157.0 male > 5 wild 5.90 148.0 male > 6 wild 5.95 148.0 male > 7 wild 8.55 160.5 male > 8 wild 5.90 148.0 male > 9 wild 8.45 161.0 male > 10 wild 4.90 147.0 male > 11 wild 6.80 153.0 male > 12 wild 5.75 146.0 male > 13 wild 8.60 160.0 male > 14 captive 6.85 159.0 male > 15 captive 7.00 160.0 male > 16 captive 6.80 155.0 male > .. > ... > 283 site 4.10 130.4 female > 284 site 3.55 131.1 female > 285 site 4.20 135.7 female > 286 site 3.45 128.0 female > 287 site 3.65 125.3 female > > The goal of my analysis is to work out what effect the categorial > factors(origin, gender) on the relation between > log(wt)~log(pes)(-->Condition, fett ressource), have. > Does the source(origin) of translocated animals have an affect on > performance(condition)in the new area? > I have already a best fit model and it looks quite good (or not?see below). > > two slopes(gender difference)and 6 intercepts(3origin levels*2gender levels) > > lm(formula = log(wt) ~ log(pes) + origin + gender + gender:log(pes)) > > Residuals: > Min 1Q Median 3Q Max > -0.54181 -0.07671 0.01520 0.09474 0.28818 > > Coefficients: > Estimate Std. Error t value Pr(>|t|) > (Intercept) -7.39879 1.97605 -3.744 0.000219 *** > log(pes) 1.78020 0.40118 4.437 1.31e-05 *** > originsite 0.06572 0.01935 3.397 0.000781 *** > originwild 0.07655 0.03552 2.155 0.032011 * > gendermale -9.32418 2.37476 -3.926 0.000109 *** > log(pes):gendermale 1.90393 0.47933 3.972 9.06e-05 *** > --- > Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 > > Residual standard error: 0.1433 on 281 degrees of freedom > Multiple R-Squared: 0.7227, Adjusted R-squared: 0.7177 > F-statistic: 146.4 on 5 and 281 DF, p-value: < 2.2e-16 > > When plot this model I get a banana-shape in Normal Q-Q Plot(with open > site pointing downwards) , indicating non-normality of my data....how > to handle this? > > -->Do I have unbalanced data? > captive site wild > n--> 119 149 19 > > My problem is that I see that my data is not as good as the > modelsummary tells. > Should I include another term in my model formular? > > I think I have to differenciate more, but I don't know > how.(contrasts?, TukeyHSD?,Akaike Information Criterion? or lme())to > many different ways out there. > > Cheers, > Tobi > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
