It's not always easy to measure what matters. Sometimes, the thing that matters doesn't make it easy for you to measure it.
The easiest path is to find a stand-in for what you care about and measure that instead. For example, websites don't actually care about how many minutes someone spends on the site, they care about transactions or ad sales or making content that moves people to take action. But those things might be harder to measure at first, so they focus on minutes.
The problem with stand-ins is that they're almost always not quite right. The stand-in looks good at first, but then employees figure out how to game the system to make the stand-in number go up instead of the thing you're actually trying to change.
A good way to find out: If you had to choose between increasing the stand-in stat and increasing the thing you actually care about, which would you invest in?
Roses, chocolates and greeting cards are a stand-in for actual human emotions, a stand-in for caring and respect and love. But of course, it's way easier to make the expense on chocolate go up than it is to actually care more.
Political fundraisers use money as a stand-in for votes, and in the short run, it might be. But not forever.
Authors use bestseller lists as a stand-in for making an impact, and in the short run, it might be. But of course, one thing is a lot easier to game than the other.
The moment you start heavily investing in making a stand-in number increase, it's worth taking a minute to look at the big sign hanging over your desk (you do have a big sign, right?) that says what you're actually seeking to do, the change you're working to make. Make that go up, even if you don't have an easy stand-in handy.