Exactly How Accurate IS Google Trends for Websites?
Much has been made of the week-old announcement that Google is in the traffic trending game. I weighed in earlier this week at ClickZ, focusing mostly on ways you can benefit from the information and largely sidestepping the already-trodden issues of Google being the only company able to opt out of the reporting, etc.
One question that hasn't been discussed to death, however, is the actual accuracy of the traffic numbers that Google is reporting. I ran some numbers on some sample sites and laid the Google Trends lines over the actual traffic numbers:
Example 1:
Example 2:
The verdict? In general, Google doesn't do too awfully bad, especially considering that neither of the sites above use Google Analytics or Urchin to measure their traffic.
The peaks and valleys are roughly similar. Roughly. Yet the scale is off pretty dramatically, with Google underreporting the traffic on one of the sites by a factor of two.
So my recommendation is that to gauge large trends (seasonality, results of large offline campaigns, etc.), Google Trends is a decent first look. It's probably a safe bet that when you plot two sites within the same vertical, that their relative lines will be more or less accurate when contrasted. But don't trust it for raw numbers.
Just to be fair, Google never said it was 100% accurate, stating in the post that "because data is estimated and aggregated over a variety of sources, it may not match the other data sources you rely on for web traffic information."