MCMC tutorial and stellar ages

I spent my research time today working on finishing the instruction manual on MCMC that Foreman-Mackey and I are writing as part of my Data Analysis Recipes series. Our goal is to get this posted to arXiv this week. I enjoy writing but learned (once again) in the re-reading that I am not properly critical of my own prose. It really takes an outsider—or a long break—to see what needs to be fixed. The long-term preservation of science is in the hands of scientists, so the writing we do is important! Anyway, enough philosophy; this is pedagogy, not research: I am trying to make this document the most useful thing it can be. I also read and commented on a big new paper by Anna Y. Q. Ho on red giant masses and ages measured in the LAMOST project. Her paper includes an incredible map of the stellar ages as a function of position on the sky, and the different components of the Galaxy are obvious!