On Sun, 14 Feb 2010, Ravi Kulkarni wrote:
Hello,
I have data for an ANOVA where the between-subjects factor has three
levels. How do I run a test of normality (using shapiro.test) on each
of the levels of the factor for the dependent variable separately
without creating extra datasets?
You can use tapply(y, x, shapiro.test) which will then conduct as many
Shapiro-Wilk tests as x has levels (without adjusting for multiple
testing).
Another approach might be to look at shapiro.test(residualslm(y ~ x)))
which tests the null hypothesis that the residuals in all groups come from
the same normal distribution.
A worked example for the chickwts data is included below.
Z
## data
summary(chickwts)
## linear model and ANOVA
fm <- lm(weight ~ feed, data = chickwts)
anova(fm)
## QQ plot for residuals + Shapiro-Wilk test
shapiro.test(residuals(fm))
## separate tests for all groups of observations
## (with some formatting)
do.call("rbind", with(chickwts, tapply(weight, feed,
function(x) unlist(shapiro.test(x)[c("statistic", "p.value")]))))
Thanks,
Ravi
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