Lisa Dowson wrote: > Hi > > I am conducting 2-sample Kolmogorov Smirnov tests for my Masters project to > determine if two independant tree populations have the same size-class > distribution or not. The trees have been placed into size-class categories > based on their basal diameters. Once I started running the stats on my data, > I got confused with the results. Just to show an example of what I was > testing I ran stats comparing population1 to population 2. and then > comparing population 3 to population 2. > Popn1 Popn2 Popn3 880 769 0 34 40 19 10 24 19 2 2 8 2 2 36 0 0 0
If I interpret your data correctly, we can look at the step functions by: plot.ecdf(x[,1], xlim=c(-10, 900), verticals=TRUE) plot.ecdf(x[,2], xlim=c(-10, 900), col.hor="blue", col.vert="blue", add=TRUE, verticals=TRUE) plot.ecdf(x[,3], xlim=c(-10, 900), col.hor="red", col.vert="red", add=TRUE, verticals=TRUE) and see that there is no good reason why the test should reject the Null for so few (6!) observations. Note that you need at least 4 observations in each group to be able to reject anything at alpha=0.05 even for completely different distributions: ks.test(1:3, 101:103) > Common sense tells me that P1 and P2 are similar and that P3 and P2 are > dissimilar. However, for the P1 versus P2 test I am getting p-value of 1 > (saying they are strongly similar -what I expected) and for the P3 versus P2 > test, a p-value of 0.9307 (saying they are strongly similar, but slightly > less similar than the other test- not what I expected at all). However, > common sense tells me that P3 and P2 are not similar at all and that I > should be expecting a far lower p-value for the test comparing P3 and P2. Sometimes common sense is dangerous. Sort your data, look again, look into the plot, and rethink... Uwe Ligges > Any help would be greatly appreciated > > Thanks > > Lisa > > [[alternative HTML version deleted]] > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
