200%?!  It doesn’t take a mathematician to reveal the absurdity in that statement.  And yet it’s true: I have seen a 200% goal conversion rate in Google Analytics with my own two eyes.

But fear not, for there is a logical explanation.  Google Analytics uses a flawed mathematical technique in calculating what it labels the “Goal Conversion Rate.”

Even the most novice Google Analytics user is likely familiar with the concept of “goals”.  GA allows you to identify key site actions as goals and measure each goal’s conversion rate (goal completions divided by site visits).  But GA also calculates what it calls “Goal Conversion Rate”, a metric that masquerades as an aggregate conversion rate for all site actions.  In reality, this metric is nothing more than the sum of the conversion rates for each individual goal.

Why is this problematic?  Well, first let’s say that XYZ.com has two primary goals, A and B.  And let’s say that all 10 visitors to XYZ.com completed both A and B.  Well, by Google’s calculations, XYZ.com’s Goal Conversion Rate would be 100% + 100%, or 200%!

But it gets even trickier when the equation doesn’t produce results that are blatantly ridiculous at first glance.  Now let’s imagine that 2 of the 10 visitors to XYZ.com complete goal A.  And let’s say that those same 2 visitors complete goal B.  Goal A’s conversion rate is 20%.  Goal B’s conversion rate is 20%.  And the website’s total Goal Conversion Rate is 20% + 20%, or 40%!  40% sounds great, but it’s a lie.  Only 2 of our 10 site visitors were goal completers–that’s 20% of our visitor base.

I could outline several more scenarios that illustrate the flawed nature of GA’s Goal Conversion Rate.  I could also explain how to manually calculate your own accurate Goal Conversion Rate of sorts.  But for now, I’ll leave you with a question: is an aggregate “Goal Conversion Rate” even a useful, actionable metric?  I’d love to hear your thoughts!

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