Resources for re-learning/reviewing relevant Statistics? I'm a junior pursuing an under-grad Political Science degree, and all of my professors have professed interest in me pursuing a PhD in political science after I've told them I would like to do so.

However, my math skills are very sub-par; I took one Statistics course my freshman year, in which I learned very little and received the class average (C-; by far the lowest grade I've received in undergrad so far)

I have a 3.9 in all my qualitative Poli Sci classes; so I'm trying to figure out how to fill this gap before Grad School.  Any advice will be appreciated!