GMAT Critical Reasoning (CR) | Public Health Official : After several years of vaccinating

Hi Mike,

I have been your keen follower.
The above question is of the 650-700 level.I just wanted to understand in a bit detail about how GMAC actually fixes the level of the question.I mean is it through some sort of Percentile analysis related to that question.

Thanks for responding in advance.

Regards,
KB