Let's say the assignment is to measure the performance of x number of
people doing the same job at different companies over a given period
of time. The problem is you did not question the survey group at the
start of the project cycle to establish a personality profile of each.
How can you credibly draw conclusions from the answers to specific
questions at the end of the cycle when those same questions were not
asked at the start of the cycle?

Ok, I'm not a Google researcher (and even if I were, I'd say $2 for a
research methodology consult would be a record low).  But... as it
happens, I just picked up a book that tackles just this sort of
general dilemma: designing evaluation research when you're faced with
the unfortunate but all-too-common reality that there was no pre-test
done of your subjects.

Here's the cite: 
Bamberger, M., Rugh, J., & Mabry, L. (2006). Real world evaluation.
Thousand Oaks, CA: Sage Publications.

It's fairly "user friendly" even if you don't have a formal background
in evaluation research or much prior research methods training.