What are the four limitations or challenges that recommendation systems sometimes face?

1. How is a recommendation engine different from a search engine?
2. Besides e-commerce websites that sell products, what are some other ways that recommendation engines are being used on the Web today?
3. What are some examples of user information required by recommendation engines that use collaborative filtering?
4. Before implementing a content-based recommendation engine, what kind of information would
website operators need to collect about their products?
5. What are the four limitations or challenges that recommendation systems sometimes face?
6. What is a recommendation engine called that combines different methodologies to create recommendations? What are three ways these systems combine methodologies?