Over the last few years I have been working towards creating a body of knowledge which helps people understand the field of Rational Level Design / Rational Design.
Rational level design is a way of objectively quantifying elements of user experience in order to create a consistent game play experience. RLD is most commonly used to understand how various game elements impact of difficulty. As difficulty plays such a significant role in determining user experience, we can use the objective, number driven system of RLD to craft user experience. Although RLD is now used to create much more than just game levels, the RLD tag has stuck and is now used to describe design activities using this data driven approach.
Gamasutra has published my most recent RLD article which is part of a larger series specifically relating to level design. The article looks at how graphing theory can be used as a tool to develop game spaces. You can check it out here