[R] Re-order levels of a categorical (factor) variable

Ravi Varadhan ravi.varadhan at jhu.edu
Wed Jan 21 23:37:28 CET 2015

I have a fairly elementary problem that I am unable to figure out.  I have a continuous variable, Y, repeatedly measured at multiple times, T.  The variable T is however is coded as a factor variable having these levels: c("Presurgery", "Day 30", "Day 60",  "Day 180", "Day 365").
When I plot the boxplot, e.g., boxplot(Y ~ T), it displays the boxes in this order:  c("Day 180", "Day 30", "Day 365", "Day 60",  "Presurgery").
Is there a way to control the order of the boxes such that they are plotted in a particular order that I want, for example:  c("Presurgery", "Day 30", "Day 60",  "Day 180", "Day 365")?

More generally, is there a simple way to redefine the ordering of the categorical variable such that this ordering will be used in whatever operation is done?  I looked at relevel, reorder, etc., but they did not seem to be applicable to my problem.

Thanks for any help.


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