The Dragon of The Source Code is virtual. He lives in the dark. He is invisible. He cannot be measured. However the results of his business are material. For instance if a program frequently crashes, the presence of errors is a clear fact.
Virtual Dragon cannot affect the material source code directly. To introduce errors in a program he needs an agent. Of course the agent of the Dragon of the Source Code is the developer. As an agent of a chemical reaction, the developer transforms the virtual presence of Dragon into bad decisions. As secret agent he destroys the quality of the software and prepares the intervention of the Dragon of The Source Code.
To study the Dragon we need consider the place when virtual becomes material. This is the developer's head.
Of course this is not the simplest place for measurements. It is complex to examine how bad decisions are born in a real software project. We'll begin from the model we can manage. We need a device to measure too.