Very concrete. That said, I think it's extraordinarily difficult to trace every step in progress back to its cause. Especially when we're talking about something like "therapy." Because what's included in that? Just the hour you spend blathering? Something specific your therapist said/did? The application of a skill learned during a session or insight made in the therapy office? The exposure homework done at the suggestion of the therapist? What if you can attribute progress to the fact that you just feel a little better about yourself, and that came from forcing yourself to get dressed, get out of the house, and go somewhere--anywhere--nevermind that it happened to be therapy? Or maybe you can attribute progress to feeling more secure, and that came from not being rejected by your therapist when you talked about a completely unrelated secret?

For me, progress isn't difficult to measure--It's the causal factor that I'm less sure about. Mostly, I just try not to look a gift horse in the mouth.