I am trying to learn Cox proportional hazard model but I have hit a wall with the `basehaz` function. 

Lets suppose for example I have some data that I want to use but there is a column such as BMI. The BMI has 3 levels: Underweight, normal weight and fat. How could one of these levels be set to zero?

In addition lets suppose I have a continuous variable such as IQ. Does that mean the model wants to take the base for the IQ to be 0?