Ecommerce
Transaction
Also called purchase, order.
Transaction is one completed order recorded with an ID and a value. A cart is not a transaction until payment clears.
How it is measured
Send one event per completed order, carrying its ID and amount, after the payment is confirmed. The order ID lets you spot duplicates when a customer reloads the confirmation page.
Tarsier counts the events it receives, so de-duplicating is your job. Fire the event once, from the server if you can, and not from a thank-you page that people refresh.
Worked example
A ticketing site records 1,240 transactions in a week while the payment provider shows 1,205. The 35 extras are confirmation-page reloads and two test orders from a staging key.
The developer moves the event to the server callback and blocks test keys. The next week the two numbers match.
How it differs
A transaction is the order. Revenue is the sum of them. A refund reverses one. Average order value divides revenue by their count. A subscription renewal is a transaction too, which a first-purchase report can hide.
Common errors
Missing order IDs. Firing on page load. Counting free or test orders. Not recording currency. Mixing live and staging data.
In practice
Reconcile tracked transactions with your payment provider every week until the two numbers stay equal for a month.