Everyone’s heard the adage that you manage what you measure, and so we all understand that it’s important to pick business metrics that encourage the right kind of growth.

But this recent retrospective on YouTube’s recommendation engine change from serving up videos with lots of views to serving up videos with lots of view time — a subtle but important difference — highlights how much metrics impact product design and your relationship with your customers.

The metrics you choose are, in fact, scope defining: if YouTube weren’t tracking how many minutes into videos people were watching (metric!), then it might not have occurred to them to re-structure their algorithm to weight it more heavily.