Extremely interesting article! I have a feeling that pull requests are now in much higher use than they used to one year ago. Can your data support such a claim, i.e. a trend to/an increased awareness and usage of pull requests?
"While pull request usage is increasing overall, par-tially reflecting Github’s growth, the relative number of reposito-ries using the pull request model has decreased slightly."
This could easily be due to Alice's using GitHub for her own master's thesis and stuff ... :) Very unlikely to receive a pull request for these kinds of projects.