Different studies of the global pitch angle of the Milky Way's spiral arms

There are many published values for the pitch angle of individual spiral arms, and their wide distribution (from -3 to -28 degrees) begs for various attempts for a single value. Each of the four statistical methods used here yields a mean pitch angle in a small range, between -12 and -14 degrees (table 7, figure 2). The final result of our meta-analysis yields a mean global pitch angle in the Milky Way's spiral arms of -13.1 degrees, plus or minus 0.6 degree.
18 pages; 2 figures, 7 tables, 1 appendix; accepted on 2015 April 14, by Monthly Notices of the Royal Astronomical Society (in press)