Your question is Testing a Modeling Assumption. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You are about to use a historical return assumption in a financial model, and you want to check whether the assumed average is supported by observed data. Before using it, you need to decide if the assumption is reasonable.
How do you test whether a financial assumption is reasonable before using it in a model?