R单向方差分析（仅获得1个DF并期望2个DF）

Question

I'm working through the examples of One-Way ANOVA on the UCLA website http://www.ats.ucla.edu/stat/r/faq/posthoc.htm . When I run the command a1 <-aov(write ~ ses) , my output differs from the example output. I'm particularly bothered by the fact that when I run the command summary(a1) , my DF on ses is 1 and there are three ses categories (1,2,3) so I'm expecting 2 DFs which is what the example on the website shows. I've checked the data for the 'write' column and 'ses' column and the counts and averages seem to match with the example, but the result from aov(write ~ ses) doesn't. Has something changed? Why am I getting only 1 DF.

hsb2 <- read.table("http://www.ats.ucla.edu/stat/data/hsb2.csv", sep=",", header=TRUE)
a1 <- aov(write ~ ses, data = hsb2)
summary(a1)
#              Df Sum Sq Mean Sq F value Pr(>F)   
# ses           1    770   769.8   8.908 0.0032 **
# Residuals   198  17109    86.4         

Answer 1

The page you are learning from has an error, in that it doesn't tell you how to enter the data correctly. The ses variable is supposed to be a factor, as we can see from the data they give us, it is read in as numeric:

str(hsb2$ses)

If we convert it to a factor, we get the same answer as the example:

hsb2$ses <- as.factor(hsb2$ses)
a1 <- aov(write ~ ses, data=hsb2)
summary(a1)

             Df Sum Sq Mean Sq F value  Pr(>F)   
ses           2    859   429.4    4.97 0.00784 **
Residuals   197  17020    86.4                   
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

In addition, using attach is highly discouraged by most R users.