Note that is a purely mathematical concept, and does not need to represent any type of ``noise-only" condition or realistic data or such. The reader, at this point, should have a bit of healthy skepticism. It should ring an alarm bell, that PDF projection is based on defining this seemingly arbitrary reference hypothesis. These concerns will be addressed two ways. First, in Section 2.2.2, we will discuss how and can be selected jointly to approximate the condition of sufficiency, and thereby approximate as close as possbile the distribution of under some arbitrary hypothesis . Later, in Chapter 3 we will discuss how can be selected according to the principle of maxumum entropy (MaxEnt), and why this is a good thing to do. For now, let's just assume that is given. Normally, must be derived analytically. Only in rare situations can can be estimated. We discuss type of problem in Section 2.2.3.