It is critical to know how any web metrics package calculates its numbers, even Google. You cannot assume, no matter how big the company, that the numbers will be correct. 

Google Analytics is probably the most popular web analytics tool in use today. It is certainly the best tool you can get for the price -- it's free and worth every penny. Unfortunately, like every web analytics tool, it is not 100 percent accurate. There are reasons why it is impossible to build perfectly accurate web analytics software, which I have covered previously in "Things that throw your stats," so we should not blame Google for this. However, Google Analytics is different from other products in that it has been intentionally designed by Google to be inaccurate over and above the normal inaccuracies that are inevitable. These inaccuracies are so glaring that most people are getting a very false picture of what is happening in their sites. This article will explain where these inaccuracies lie and provide methods to correct for them.

Visits and bounces
Firstly, we must remember that Google did not create Google Analytics.  The company bought a product called Urchin and rebranded it. After about a year, it then released an upgraded version. Urchin had contained a very big mistake in its calculations, which Google fixed in the upgrade. The problem lay with counting visits. According to all the web analytics standards, a "visit" only happens when someone reads more than one page on a site. If someone comes to your site, looks at the first page, then leaves, this is not a "visit" -- it is a "bounce." The best way to think of this is in terms of a shop. If someone looks at the store window but moves on, they have bounced. It is only when they enter the store (or site) that you have a visit.

This is a very important definition to grasp because it fundamentally affects how you see the performance of the site. The reason we need to distinguish between bounces and visits is that we can't tell how long someone spent looking at a web page. We can only tell what time they accessed it. We calculate the time spent looking at a page by comparing what time they accessed it with what time they accessed the next page.

For example, if someone accesses my homepage at 2 p.m., then accesses the next page at 2:15 p.m., we assume they spent 15 minutes reading the homepage. If someone only accesses one page -- in other words, if they bounce -- we have no way of knowing how long they spent looking at that one page. So you can only calculate visit duration when someone reads two or more pages.

A key metric when analyzing the performance of a site is the average duration -- how long people are spending in the site. You need to know your overall average duration for the site, and you often want to know the average duration for visits from certain ads or for specific products. It's a very good way of assessing engagement. Average duration is calculated by totaling up all the individual visit durations and dividing this by the total number of visits.

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