To be considered a sack the quarterback must intend to throw a forward pass. If the play is designed for the quarterback to rush the ball, any loss is subtracted from the quarterback's rushing total. If the quarterback's intent is not obvious statisticians use certain criteria, such as the offensive line blocking scheme, to decide. Other unique situations where a loss reduces a quarterback's rushing total (not a sack) are "kneel downs" (used to run time off the game clock), and aborted plays, such as a fumbled snap that the quarterback falls on to maintain possession.
A player will receive credit for half of a sack when multiple players contribute to the sacking of a quarterback.