Hi all, I have a very easy questions (I hope). I had measure a property of plants, growing in three different substrates (A, B and C). The rest of the conditions remained constant. There was very high variation on the results. I want to do address, whether there is any difference in the response (my measurement) from substrate to substrate?
x<-c('A','A','A','A','A','B','B','B','B','B','C','C','C','C','C') # Substrate type y <- c(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15) # Results of the measurement MD<-data.frame(x,y) I wrote a linear model for this: summary(lm(y~x,data=MD)) This is the output: Call: lm(formula = y ~ x, data = MD) Residuals: Min 1Q Median 3Q Max -2.000e+00 -1.000e+00 5.551e-17 1.000e+00 2.000e+00 Coefficients: Estimate Std. Error t value Pr(>|t|) (Intercept) 3.0000 0.7071 4.243 0.001142 ** xB 5.0000 1.0000 5.000 0.000309 *** xC 10.0000 1.0000 10.000 3.58e-07 *** --- Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 Residual standard error: 1.581 on 12 degrees of freedom Multiple R-squared: 0.8929, Adjusted R-squared: 0.875 F-statistic: 50 on 2 and 12 DF, p-value: 1.513e-06 I conclude that there is an effect of substrate type (x). NOW the questions : 1) Do the fact that the all p-values are significant means that all the groups are different from each other ? 2) Is there a (easy) way to plot, mean plus/minus 2*sd for each substrate type ? (with asterisks denoting significant differences ?) THANKS ! version platform x86_64-apple-darwin9.8.0 arch x86_64 os darwin9.8.0 system x86_64, darwin9.8.0 status major 2 minor 11.1 year 2010 month 05 day 31 svn rev 52157 language R version.string R version 2.11.1 (2010-05-31) ______________________________________________ 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.