  Open Source Software (OSS) project success relies on crowd contributions.
When an issue arises in pull-request based systems, @-mentions are used to call
on people to task; previous studies have shown that @-mentions in discussions
are associated with faster issue resolution. In most projects there may be many
developers who could technically handle a variety of tasks. But OSS supports
dynamic teams distributed across a wide variety of social and geographic
backgrounds, as well as levels of involvement. It is, then, important to know
whom to call on, i.e., who can be relied or trusted with important task-related
duties, and why.
  In this paper, we sought to understand which observable socio-technical
attributes of developers can be used to build good models of them being future
@-mentioned in GitHub issues and pull request discussions. We built overall and
project-specific predictive models of future @-mentions, in order to capture
the determinants of @-mentions in each of two hundred GitHub projects, and to
understand if and how those determinants differ between projects. We found that
visibility, expertise, and productivity are associated with an increase in
@-mentions, while responsiveness is not, in the presence of a number of control
variables. Also, we find that though project-specific differences exist, the
overall model can be used for cross-project prediction, indicating its
GitHub-wide utility.
