Summary: Your BCBA picks the assessments and reads the results. Your job is the hands-on part: prep the materials, follow the protocol to the letter, take clean data, and report back what you saw. Get those right and the assessment is valid. Cut corners and the BCBA ends up making decisions from bad data, which is the one thing assessment is supposed to prevent.
Here's the division of labor, and the exam tests it constantly. The BCBA selects, designs, and interprets assessments. You assist. You prep, you administer under their direction, you collect data, you report what happened. You don't decide which assessment to run, and you don't interpret the scores. If you keep that line straight in your head, most of the scope-of-practice questions on this topic answer themselves.
Most of conducting assessments isn't glamorous. It's reading the protocol carefully, setting up the room, and writing down exactly what the kid did. But that boring discipline is what makes the data trustworthy, and trustworthy data is the whole point.
Getting ready before the session
Walking into an assessment unprepared is how you waste a client's session and your supervisor's time. Do the boring prep first.
Know the protocol cold. Read the procedure before the client is in front of you, not while they're waiting. You need to know how to present each item, what counts as a correct response, what counts as incorrect, and what the timing rules are. If anything is unclear, ask your BCBA before the session, not after you've already run ten trials wrong.
That's the free preview of Conducting Assessments. The complete guide, plus all 851 practice questions and 3 full exams, is in the study pack and the book.