Dick Grote in The Myth of Performance Metrics:

Insisting that there must be quantifiable metrics can lead us astray in accurately evaluating performance. For example, how would you evaluate the performance of a translator? The obvious, easy, and wrong answer: the number of documents translated. But while that aspect of performance is easily quantified, it ignores what’s genuinely important — the ability to capture nuance.

And if you think about it, that’s what we all are: translators.