Students should check their admission letter to make sure that they are fulfilling the requirements for funding. Continued funding is contingent upon satisfactory progress toward the Ph.D. degree according to the Statistics Department's procedures of evaluation. Summer support is contingent upon approved proposals for summer study (in addition to satisfactory progress toward the PhD degree).
Graduate funding is a collaborative effort between students and the university. Because the mentored experience of writing grant applications as a graduate student is irreplaceable, and the receipt of external awards constitutes a significant advantage on the job market, doctoral students who receive more than two years of guaranteed University appropriated funding (as opposed to funding from externally sponsored projects) are required to apply for external grants no later than the fourth year of their PhD program. Students are not required to win an external award to fulfill this requirement – only to apply.
Regulations governing recipients of University assistance are described at http://www.tgs.northwestern.edu/funding/index.html