Bird and I spent our last hours together in Heidelberg working out a plan and to-do list for his paper on the age–velocity relation in the Milky-Way disk. We planned tests and extensions, the scope of the minimal paper, and a visualization of the model. We worked out a way to express the model in the space of the raw-ish data, which is important: You can't assess what your model is doing in the space of the latent parameters; the data are the only things that exist (imho). That is, you have to project your model into the data space to assess where it is working and not working. And to decide whether you need to do more. That's old-school Roweis.