To what extent is the writer using an emotional appeal or a rational appeal. The most persuasive argument is the one that supports its thesis by referring to relevant, accurate, and up to date data from the best sources possible.
Empiricism implies going to the best source for material.
This suggests that original material is preferable to secondary sources, which is preferable to hearsay. Another aspect in determining the empirical quality of an argument is understanding any hidden bias of the author. This means that you will need to search out the assumptions behind the writer's work.