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 ______________________________________________ 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.