Hi, I have been watching the forums in the past few days. Nice to see people have some questions. I clarified some questions today in the class and later by individual appointments. First of all you need to state your assumptions. After interaction with multiple students, here are some of the ways that you can approach the terms mentioned in the case study. "Each eye surgeon has one or two minor surgeries in which they work". Some of you are considering minor surgery as a service. This could work, but remember that in such clinics, surgery is often referred to as a "surgery room" as indicated in the case study "...in which they work". Just state your assumption when you are referring to this term. "a parent might be a customer without being a patient if they pay the bills for their child who is a patient." Hint: One option is to do specialisation here. For instance for a class person, you can have two subclasses, patient and customer. You need to think of how you are going to apply constraints. "the patient cannot have multiple appointments with the same surgeon on the same day" Hint: Think of how you can make a tuple unique. Treatment vs Service. Hint: Assume eyesight-check is a treatment, it may offer finding the right power for the classes and may also offer fitting contact lenses. Cheers Mehwish