...week moving around of traffic levels is normal. Nonetheless, you should be able to get a rough timeframe on when the problemstarted. Next, see if the downturn is affecting your site in a broad based manner, or if it is isolated to a section...
http://searchengineland.com/how-to-handle-an-seo-setback-24052
...to backtrack data patterns before and after any change that might be a problem’s root cause to identify when the problemstarted. It is also able to provide data to prove the value of best practice such as avoiding deployment of changes at a time...
http://www.sumerian.com/downloads/Butler_ITAnalytics_whitepaper.pdf