Product Management - How to Measure Customer Metrics

How do you measure behavior changed, and whether it's been successful or not? The basi way to do it is an experiment. If we do three versions of this, which one were to get people to do acts, that would be the gold standard. When i was a product manager, we went from a world where we were measured on engagement - how frequently did people use the product? This is exactly the wrong metric to measure.