A field study was a period undergone by cadets at Starfleet Academy during their sophomore year which was in practice in 2373. Field study involved active duty aboard a Starfleet vessel, space station, or other installation during which the cadet was placed in a rotation of various posts, ostensibly to gain practical experience. It was similar to a training cruise, but more routinely involved active-duty situations among active-duty officers as opposed to environments and training ships meant primarily for education.
Nog's field study was performed on Deep Space 9 in 2373. Captain Sisko told him "The key to field study is simple. Keep your eyes and ears open, follow orders and try not to get in the way". (DS9: "The Ascent")