I agree with all the aforementioned comments that all these roles are crucial for a product manager. The one point that I have to elaborate on is the ability of the product manager to understand the factors that could affect sales. The product manager does not need to only understand those factors and prevent them, but he/she should work around the clock to ensure that the product is living up to the quality promised to the customer. Product managers should always work on continually improving their products and ensure that the product meets all regulatory requirements. Since there is no system that robust 100% of the time, the product manager have to make sure that all inadvertent defeciencies are eliminated and that there is enough product in stock to meet the demand.