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# Analysis of variance

Analysis of variance (Anova) is used to fit data to a linear model when all explanatory variables are categorical. Each of these categorical explanatory variables is known as a factor, and each factor can have two or more levels. When a single factor is present with three or more levels, we use a one-way Anova to analyze the data. If we had a single factor with two levels, we would use a student's t-test to analyze the data. When there are two or more factors, we would use a two-way or three-way Anova. You can easily perform an Anova in R using the `aov()` function.

In the first example, we will look at the effect of the dose of `drugA` on the level of fatigue reported by the 20 patients as follows:

`> patient.fatigue <- read.table(header=TRUE, ...`

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