First off, here's the official documentation: https://support.google.com/adwords/answer/42995?hl=en
Clicks on ads that Google considers to be illegitimate, such as unintentional clicks or clicks resulting from malicious software.
We did some analysis of invalid clicks for an account we were optimizing:
As you can see, on some days, invalid clicks make up a large chunk of the total number of clicks.
Generally, this is no big deal, because google doesn't charge you for clicks that aren't legitimate.
When you should you should care
1. If you're using the AdWords API
The adwords reporting api has a number of "reports" you can query from. The
CLICKS_PERFORMANCE_REPORT is special because it includes data from invalid clicks. If you sum the number of clicks in a day from the
CLICKS_PERFORMANCE_REPORT and compare it to, say, the
CAMPAIGN_PERFORMANCE_REPORT, you will get a different number of clicks. The extra clicks are invalid clicks, but are not flagged in any special way in the report. There's no way of knowing.
This means that the
CLICKS_PERFORMANCE_REPORT is no good for any kind of aggregate data gathering. It is only good for looking up a specific GCLID (that you know is legitimate) to get a specific click's data.
2. If you're building some custom user tracking software
A good chunk of invalid clicks still get to your landing page. Since they have a GCLID (
/your-landing-page?gclid=<long_randomish_string>), tracking software might tag them as coming from a PPC channel, even though you got them for free. This is problematic later if the software tries to derive CPCs and other statistics from the number of GCLIDs it has collected.
At Altus, we have found no solution to this problem in our custom tracking projects. We built our tools around the assumption that the number of clicks tracked will be off by a small percentage.