Just because you can measure something, doesn’t mean that it is a good proxy from which to draw a conclusion.
In the case of web sites, one proxy used to measure “success” is an increasing number of page views over time. But if you redesign your site to make it easier for visitors to find what they are looking for, page views (per session) may decline. That would be a good thing, although it’s counter to the belief that more is better.
Turning to cancer, a similar counterintuitive proxy is “survival rate.”