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Use date comparisons to identify not just a change, but a change in the change.
You updated your home page at the beginning of April 2013; otherwise, you made no other changes to your website. When you check Google Analytics a month later, you see that the conversion rate for your primary goal was 10% in April.
You know that this number is not very meaningful if not compared to another time period, but which time period should you compare it to?
Compare it to March 2013?
You see that the site had an 8% conversion rate in March, so the home page update was effective, right? Well, not necessarily. If conversion rate increased by 2% from March to April during previous years as well, 2013 would only be following the seasonal trend.
Compare it to April 2012?
You see that the site had a 9% conversion rate in April 2012, so the update to the home page must have driven those extra conversions, right? Well, again, not necessarily. Conversions could have already been trending up during the previous months, so the home page update could have had no effect or even a negative one.
What you really need to compare is conversion rate over the course of March and April 2013 vs. March and April 2012. In this way, you’re not just identifying a change relative to the previous month or the same month of the previous year – you’re identifying a change in the change.
To complete the picture, we see that the conversion rate in March 2012 came in at 8%, just like March 2012. That means that the increase from March 2013 to April 2013 was 2% – 1% greater than the increase from March 2012 to April 2012.
Now you can celebrate – the home page update very likely increased conversion rate. (But next time, test the new home page in a Content Experiment to be sure!)