bring in X now we might have a model for how X is related to Y so you'll have px given Y and so this equation here is one that we'll find ourselves using a lot one of the two equations we'll be using over and over same thing can of course be done for densities and densities this summation would become an integral because you have to integrate over all continuous barrels values the variable can take on whereas in some in discrete variables you just sum otherwise same thing though in general whenever there's a summation you can put an integral same