With Google Analytics, you do not have the option to reprocess data like you do with log file analytics systems, so it's important that you are cautious with the changes you're making to your profiles.

If you set up a new filter incorrectly and no one notices for two weeks that you are collecting the wrong data, there is no way to go back and reprocess that data. It's gone and it's not coming back. Because of this, it is highly recommended that you make your filter, goal, and/or funnel changes to a test profile before making them to your main profile. By collecting the results in a test profile, you can gauge whether or not you are receiving the outcome you anticipated.

Likewise, if you create a new filter or goal, it will only affect the data from that point moving forward. For example, if you only want to include visitor traffic to a specific subdirectory, your historical data will still include visitor traffic to the rest of your site. This can also make things tricky with goals and funnels.

Also, if you create a new profile for an existing domain, you will not have access to historical data within that new profile. Profiles only collect data that is processed after they were created, so you may want to create several profiles at once if you think you may want to use the historical data in a later profile.

Recently, Google Analytics had introduced a new feature called Benchmarking, where you can see your stats compared to others' in your industry. If you decide to opt-in to data sharing, Google Analytics will include historical data up to a month before your opt-in date.

And for those of you who haven't switched over from urchin.js yet, historical data will be kept when you move your site over to ga.js, so don't worry.

So don't forget to test before you implement anything new to your main profile, and consider keeping a change log of the changes to your profiles. Remember that any changes you make won't change your historical data.

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