As a software tester, you need to know what the proxy measurement is because this type of measurement is a widely used opportunity for disaster. The post is aligned with the Black Box Software Testing Foundations course (BBST) designed by Rebecca Fiedler, Cem Kaner, and James Bach.
There are things around us that could not be measured because we do not know how to measure it. Then we try to correlate this not measurable attribute with another attribute that we know to measure. We call this the proxy (or surrogate) measure.
This is an opportunity for disaster because there is no relation between attribute and surrogate attribute.
We measure person goodness by the number of donations that a person made last year.
We measure software tester productivity by counting the reported bugs.
In the first example amount of donations is surrogate for goodness, and in the second example, the count of bugs is surrogate for productivity.
The problem is that people optimize for what we measure at the expense of what we do not measure. In bug counting, you will have a high number of low quality bug reports that would not reveal the actual status of testing.
Problem with donation counting is that Donald Trump motivation for a donation could be a tax deduction, while some person that unselfishly helps her senior neighbor could donate zero dollars. Who is a better person?