One important thing to keep in mind when measuring click through rates are that the "impression" values as well as the "click" values can often be heavily skewed by accident. For example, Google sends bot traffic to evaluate any new ads you put up on Google AdWords. You aren't billed for these clicks, nor do they show up in AdWords reporting, but if you have any third party reporting, they may pick these up as "visits" or "clicks" by accident, which can really throw off your metrics. This was a common problem with a company I worked for, who also had a homegrown reporting system. I was constantly having to explain this to them.