What Ecommerce Data Do You Think Is the most important to have?

Hello webmasters,
If you are managing an ecommerce store, and you want to analyze data from your online store. What data do you think would be the most important data you want to know right away?

In common, I would say ROI (return on investment).
Because it's always about how much time/money/effort you put in and get back.

There are several KPIs depending on the actual situation of your store.
For me it was the best way to align the whole path with all the different metrics.

e.g.
- You need the right(!) traffic for turnover, therefore you need to monitor which channel brings the most-valuable-traffic with lowest efforts.
- most-valuable-traffic could be based on conversion-rate, value per order or customer-life-time-value
- customer-life-time-value is based on number of orders, average value per order, average return rate, etc.

Then you could calculate if it's cheaper to research & adapt page layout to double your conversion rate or if you only have to double your spendings in paid traffic.

Source of traffic, device types, browsers, average time on site, etc., (if users click the back button and they go back from where they came - search engine - you will be amazed how many click the back button)