How to convert Wide format data to Long format data (process called 'reshaping')is very very important to understand. Certain statistical tests cannot be performed if not in Long format.
The article mentions Reshape2. I also love TidyR. It's much easier to use than Reshape2 when dealing with a large number of variables (i.e., columns).
Interesting, I'll have to watch out for it then. I found it more helpful than Reshape2 when trying to convert a dataset with 55 variables from wide to long format. 8 of those variables needed to be collapsed into 1 variable. With Reshape2, I had to identify all the ones I didn't want in the final result, with TidyR I just needed to identify the ones I wanted. Hence it was easier with TidyR.
The article mentions Reshape2. I also love TidyR. It's much easier to use than Reshape2 when dealing with a large number of variables (i.e., columns).