Quality
Hit inflation
Also called fake traffic.
Hit inflation is artificial volume that makes traffic or conversion numbers look larger. It does not matter whether the cause is a bug or an attack.
How it is measured
Look for hits that do not fit human behaviour: a single URL hit thousands of times, a country that never visited before, or events that fire twice per click. Compare with server logs and conversions.
A tag that fires twice is the most common cause on a normal site. Test by loading one page and counting requests.
Worked example
A SaaS site's pageviews jump from 21,000 to 43,000 a week after a theme update. Sessions rise 3 percent. The new header includes a second copy of the tracker, so every page counts twice.
Removing the duplicate returns pageviews to 21,500.
How it differs
Hit inflation is what shows up. Bot traffic is one cause. Referrer spam is another. Ad blocker effects work in the opposite direction, deflating counts.
Common errors
Celebrating a jump. Comparing pageviews alone. Forgetting duplicate tags. Missing paid-click fraud. Not tying the figure to revenue.
In practice
After any site change, load one page and count requests, then compare pageviews and sessions for the next two days.