Quality
Referrer spam
Also called ghost spam.
Referrer spam is forged hits whose only purpose is to plant a junk referrer in reports. The spammer hopes you click the link.
How it is measured
Spot them by referrers that you never linked from, zero engagement, a single page, and odd hostnames. Many send hits directly to the collector without loading your site.
A write token on the tracker makes forged hits harder, because the collector can reject requests that do not carry it.
Worked example
A small blog's referrer list adds seven new domains in a week, each with between 20 and 40 visits, one page and zero seconds. The total looks like 190 extra visits, 11 percent of the week.
The owner filters the seven domains, and the referrer report goes back to showing search, social and two newsletters.
How it differs
Referrer spam is forged. A real referrer is the page that linked. Bot traffic is the wider category. A filter removes it from reports.
Common errors
Visiting the spam sites. Blocking by guesswork. Filtering real referrers. Ignoring the pattern. Leaving a tracker with no token.
In practice
Look at the referrer list monthly, filter zero-engagement domains and keep your tracker's write token on.