Correcting Google Analytics
If I have inaccurate metrics that are being used by someone to assess performance, I like to aim for minimal disruption when I correct them. I therefore leave the existing metrics in place and add a few (accurate) ones. I have found the best way to correct for this problem is to add a metric I call "Retained Visits," leaving the existing Total Visits in place. I can then re-calculate my metrics using Retained Visits.

I get my number of Retained Visits by removing the bounces. For example, if Total Visits was 1,000 and the Bounce Rate was 25 percent, I had 250 bounces, so the number of Retained Visits was 750.

Mathematically, we express this as:

RV = TV – (TV * BR)

where:
RV is Retained Visits
TV is Total Visits
BR is Bounce Rate

Correcting average duration
In order to get the genuine Average Duration, I need to rewind the calculation Google made about half way. Google Analytics calculated Average Duration by adding up all the individual durations and then dividing that by the number of visits (including bounces). I rewind this by multiplying the Average Duration by the Total Visits. That gives me the number Google had before it factored in bounces, which we can call Total Duration. (Remember that the bounces, with durations of zero, contributed nothing to this number.) I then redo the calculation of average using Retained Visits, thus:

TAD = (AD * TV) / RV

where:
TAD is True Average Duration
AD is Average Duration (as reported by Google Analytics)
TV is Total Visits
RV is Retained Visits

It is surprising, even terrifying, how far apart the old and new numbers are, and the difference in the picture this provides. Here is a set of figures from a site that averages around 30 percent bounce rate:



These show that rather than an average engagement of around three minutes, the reality is closer to five -- people are spending almost twice as long on the site as Google Analytics says they are.

You can use Retained Visits to recalculate your Conversion Rate in a similar fashion. While I keep my calculations of AdWords ROI untouched, I also add another one based on Retained Visits. I have found that if you break down the ROI on each advertising campaign (or ad) by Retained Visits, you can often find extremely valuable revenue streams concealed inside a bland total.

Conclusion
It is critical to know how any web metrics package calculates its numbers. You cannot assume, no matter how big the company, that the numbers will be correct. A small variation doesn't make much difference, but you could be getting a completely false view of what is happening in your site. You may need to correct those numbers yourself. I recommend the use of Retained Visitors as a good way of correcting for errors in Google Analytics.

The other conclusion we can draw from this is how limited the understanding of web analytics is among practitioners, and what a low priority Google gives Google Analytics. Can you imagine an accountant insisting on using inaccurate numbers in their bookkeeping simply because that's what they had been doing before? How long do you think that business would survive? Can you imagine Google agreeing to downgrade the accuracy of its search results to meet user demand? On second thought, that's exactly what they agreed to do in China, isn't it?

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Brandt Dainow is an independent web analytics and marketing consultant working in the U.K. and Ireland.