Ecommerce
Add to cart
Also called ATC.
Add to cart is the event fired when an item is added to a cart. It is the first signal of purchase intent that you can count.
How it is measured
Fire one event when the cart accepts the item, with the product ID, quantity and price. Fire on the cart's response, not the button click, so out-of-stock items do not count. A quantity change in the cart is a separate event.
In Tarsier this is a custom event such as add_to_cart, with short data like the SKU. Events never change visitor or bounce counts, so the cart sits beside traffic, not in it.
Worked example
A pottery shop sees 1,400 add_to_cart events in a week and 120 orders. The ratio is 8.6 percent. But 310 of the adds are the same mug added, removed and added again by people comparing glazes.
The owner adds a swatch page so people stop using the cart to compare, and adds fall to 1,050 with orders at 118.
How it differs
Add to cart is the step into the basket. Product view is what came before. Checkout step is what follows. Cart abandonment is the share of carts that never turn into an order.
Common errors
Firing on click and not on success. Counting quantity changes as adds. Double-firing on both the button and the form. Ignoring adds from wishlist. Treating an add as a sale.
In practice
Test with one real product, add it twice, remove it, and confirm the log shows what you expect.