Quality
Discrepancy
Also called number mismatch.
Discrepancy is a gap between two systems counting the same activity. A gap of 5 to 15 percent is normal, a gap of 50 is a bug.
How it is measured
Pick the same period and the same definition in both systems, then compare. Common causes are blockers, bots, time zones, sessions rules and tags that fail to fire.
Tarsier will rarely match another tool exactly, because it counts without cookies and with its own session rule.
Worked example
A store's Tarsier count shows 6,200 pageviews on the day its ad platform charged for 7,100 clicks. The gap of 900 is 13 percent. Some clicks bounce before the script loads, some were bots, and some were blocked.
The store asks the platform for invalid-click credit on 300 of them.
How it differs
A discrepancy is the gap. An ad blocker, bot traffic and sampling are three common causes. It is the symptom, not the diagnosis.
Common errors
Chasing exact matches. Comparing different definitions. Ignoring time zones. Assuming one tool is truth. Skipping a test visit.
In practice
Run a test visit through both tools and write down the expected ratio. Investigate only when it moves.