Thanks a lot for your answers and reading suggestions, now I know my guess was completely wrong.
I guess in my case it will be more informative to keep the unordered factors. That way I can know not only that days differ in general, but also get information on which day is differing from day 1. Cheers; Catarina On 15 November 2011 17:54, Paul Johnson <pauljoh...@gmail.com> wrote: > On Tue, Nov 15, 2011 at 9:00 AM, Catarina Miranda > <catarina.mira...@gmail.com> wrote: > > Hello; > > > > I am having a problems with the interpretation of models using ordered or > > unordered predictors. > > I am running models in lmer but I will try to give a simplified example > > data set using lm. > > Both in the example and in my real data set I use a predictor variable > > referring to 3 consecutive days of an experiment. It is a factor, and I > > thought it would be more correct to consider it ordered. > > Below is my example code with my comments/ideas along it. > > Can someone help me to understand what is happening? > > Dear Catarina: > > I have had the same question, and I hope my answers help you > understand what's going on. > > The short version: > > http://pj.freefaculty.org/R/WorkingExamples/orderedFactor-01.R > > The longer version, "Working with Ordinal Predictors" > > http://pj.freefaculty.org/ResearchPapers/MidWest09/Midwest09.pdf > > HTH > pj > > > > > Thanks a lot in advance; > > > > Catarina Miranda > > > > > > y<-c(72,25,24,2,18,38,62,30,78,34,67,21,97,79,64,53,27,81) > > > > Day<-c(rep("Day 1",6),rep("Day 2",6),rep("Day 3",6)) > > > > dataf<-data.frame(y,Day) > > > > str(dataf) #Day is not ordered > > #'data.frame': 18 obs. of 2 variables: > > # $ y : num 72 25 24 2 18 38 62 30 78 34 ... > > # $ Day: Factor w/ 3 levels "Day 1","Day 2",..: 1 1 1 1 1 1 2 2 2 2 ... > > > > summary(lm(y~Day,data=dataf)) #Day 2 is not significantly different from > > Day 1, but Day 3 is. > > # > > #Call: > > #lm(formula = y ~ Day, data = dataf) > > # > > #Residuals: > > # Min 1Q Median 3Q Max > > #-39.833 -14.458 -3.833 13.958 42.167 > > # > > #Coefficients: > > # Estimate Std. Error t value Pr(>|t|) > > #(Intercept) 29.833 9.755 3.058 0.00797 ** > > #DayDay 2 18.833 13.796 1.365 0.19234 > > #DayDay 3 37.000 13.796 2.682 0.01707 * > > #--- > > #Signif. codes: 0 *** 0.001 ** 0.01 * 0.05 . 0.1 1 > > # > > #Residual standard error: 23.9 on 15 degrees of freedom > > #Multiple R-squared: 0.3241, Adjusted R-squared: 0.234 > > #F-statistic: 3.597 on 2 and 15 DF, p-value: 0.05297 > > # > > > > dataf$Day<-ordered(dataf$Day) > > > > str(dataf) # "Day 1"<"Day 2"<"Day 3" > > #'data.frame': 18 obs. of 2 variables: > > # $ y : num 72 25 24 2 18 38 62 30 78 34 ... > > # $ Day: Ord.factor w/ 3 levels "Day 1"<"Day 2"<..: 1 1 1 1 1 1 2 2 2 2 > ... > > > > summary(lm(y~Day,data=dataf)) #Significances reversed (or "Day.L" and > > "Day.Q" are not sinonimous "Day 2" and "Day 3"?): Day 2 (".L") is > > significantly different from Day 1, but Day 3 (.Q) isn't. > > > > #Call: > > #lm(formula = y ~ Day, data = dataf) > > # > > #Residuals: > > # Min 1Q Median 3Q Max > > #-39.833 -14.458 -3.833 13.958 42.167 > > # > > #Coefficients: > > # Estimate Std. Error t value Pr(>|t|) > > #(Intercept) 48.4444 5.6322 8.601 3.49e-07 *** > > #Day.L 26.1630 9.7553 2.682 0.0171 * > > #Day.Q -0.2722 9.7553 -0.028 0.9781 > > #--- > > #Signif. codes: 0 *** 0.001 ** 0.01 * 0.05 . 0.1 1 > > # > > #Residual standard error: 23.9 on 15 degrees of freedom > > #Multiple R-squared: 0.3241, Adjusted R-squared: 0.234 > > #F-statistic: 3.597 on 2 and 15 DF, p-value: 0.05297 > > > > [[alternative HTML version deleted]] > > > > > > ______________________________________________ > > 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. > > > > > > > > -- > Paul E. Johnson > Professor, Political Science > 1541 Lilac Lane, Room 504 > University of Kansas > [[alternative HTML version deleted]]
______________________________________________ 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.
