14.7 Repeated measures ANOVA using the lme4 package

If you are conducting an analyses where you’re repeating measurements over one or more third variables, like giving the same participant different tests, you should do a mixed-effects regression analysis. To do this, you should use the lmer function in the lme4 package. For example, in our poopdeck data, we have repeated measurements for days. That is, on each day, we had 6 measurements. Now, it’s possible that the overall cleaning times differed depending on the day. We can account for this by including random intercepts for day by adding the (1|day) term to the formula specification. For more tips on mixed-effects analyses, check out this great tutorial by Bodo Winter at http://www.bodowinter.com/tutorial/bw_LME_tutorial2.pdf.

# install.packages(lme4)  # If you don't have the package already

# Calculate a mixed-effects regression on time with
#  Two fixed factors (cleaner and type)
#  And one repeated measure (day)

my.mod <- lmer(formula = time ~ cleaner + type + (1|day),
                data = poopdeck)