Simpson's paradox

I spent part of the day working through Moe & Di Stefano (2017), which is an immense and comprehensive paper on binary-star populations. The reason for my attention: Adrian Price-Whelan (Princeton) and I need a parameterization for the binary-star population work we are doing in APOGEE. We are not going to make the same choices as those made by Moe, but there is tons of relevant content in that paper. What a tour de force!

I spent part of the afternoon crashing the RAVE Collaboration meeting at the University of Heidelberg. I learned many things, though my main point was to catch up with Matijevic, Minchev, Steinmetz, and Freeman! Ivan Minchev (AIP), in his talk, discussed relationships between age, metallicity, and Galactocentric radius for stars in the disk. He has a beautiful example of Simpson's paradox, in which, for small population slices (age slices), the lower metallicity stars have higher tangential velocities, but overall the opposite is true, causing measured gradients to depend very strongly on the measurement uncertainties (because: Can you slice the populations finely enough in age?). We discussed paths to resolving this with a proper generative model of the data.