Yang Mei Lian - 1 month ago 12

R Question

In R, I have four numerical vectors. I want to test if mean of these four vectors are the same or not, and if not, which mean is greater and which mean is smaller (basically I want to rank four vector by mean of them).

This post is quite suitable for me:

http://brownmath.com/stat/anova1

I am following this guide:

https://www.r-bloggers.com/analysis-of-variance-anova-for-multiple-comparisons/

However, this post stops at testing if four means are equal or not, but if the answer is not, how can I know which mean is greater and which mean is smaller in R?

Thank you very much,

The code from the post is:

`a = c(65, 48, 66, 75, 70, 55)`

b = c(64, 44, 70, 70, 68, 59)

c = c(60, 50, 65, 69, 69, 57)

d = c(62, 46, 68, 72, 67, 56)

dati = c(a, b, c, d)

groups = factor(rep(letters[1:4], each = 6))

fit = lm(formula = dati ~ groups)

anova (fit)

Answer

I'll assume that assumptions are fulfilled, in particular, that **residuals are independent** and that the **distribution of residuals is homoskedastic and sufficiently similar to a normal distribution**.

First note that your ANOVA returns a p-value of 0.99, which means there is no significant difference between group means. If the p-value was below 0.05, you could do a post-hoc test for pairwise comparisons.

```
TukeyHSD(aov(fit))
# Tukey multiple comparisons of means
# 95% family-wise confidence level
#
#Fit: aov(formula = fit)
#
#$groups
# diff lwr upr p adj
#b-a -0.6666667 -15.68081 14.34748 0.9992898
#c-a -1.5000000 -16.51415 13.51415 0.9921250
#d-a -1.3333333 -16.34748 13.68081 0.9944302
#c-b -0.8333333 -15.84748 14.18081 0.9986181
#d-b -0.6666667 -15.68081 14.34748 0.9992898
#d-c 0.1666667 -14.84748 15.18081 0.9999888
```

Of course, there is nothing significant here.

A bit more powerful is the multcomp package. It let's you define different pairwise comparisons. It also can give you the group letters that you often see in tables or figures.

```
library(multcomp)
pairwise <- glht(fit, linfct = mcp(groups = "Tukey"))
summary(pairwise)
cld(pairwise)
# a b c d
#"a" "a" "a" "a"
```

Again, since there is nothing significant in your example, all factor levels get the same letter.