The strata will be the category of people with at least 3 years of working experience scoring AP Statistics
Explanation:
The strata will be the category of people with at least 3 years of working experience scoring AP Statistics Exams.
I will use this strata because it is only those with a track record of scoring AP Statistics Exams that can rightly score for ETS. People who don't have the right amount of experience of scoring AP Statistics exam may need some training because they need to understand the specifics on how to grade some questions and respective answers.