In "The Ascent," in the scene wherein Nog first reports to Sisko, Sisko mentions where he had his field study. This would be a good addition, if anyone wants to look it up.
My addition to distinguish between field study and training cruises is drawn from the following reasoning. Nog's field study aboard DS9 is obviously intended to place the cadet in an active-duty situation. A training cruise is part of TWOK, and when a problem arises, it is obvious that neither ship nor crew were expecting nor completely prepared for the event. This suggests that field study is active duty, while training cruises are primarily educational. - Jephthah