We meticulously monitor all of our SEO projects for changes in their Google Analytics for clues as to what is working and what requires more attention. We often compare monthly changes by year, and at the end of the year, we like to see our progress and determine how much value we provide.
Here is a good example of one particular client's data, comparing 2012 to 2013. This is a real client and this is real data. We took over the account in April and by the end of May we had redesigned their site and were in the beginning stages of their SEO program.