Loss exchange ratio is a figure of merit in attrition warfare. It is usually relevant to a condition or state of war where one side depletes the resources of another through attrition. Specifically and most often used as a comparator in aerial combat, where it is known as a "kill-ratio." For example, during the Korean War, American combat jets had a kill-ratio of 12-1 and often higher (stated as twelve to one). This means for every twelve aircraft shot down by an American aircraft, one American plane was shot down by an enemy fighter. It was a rather good ratio for the Americans, especially when compared to some loss rates of 1-1.1 encountered by American fighter aircraft during much of the air war over southeast Asia. This loss rate of nearly one to one was a further setback for the American forces when considering the more sophisticated American aircraft were many times more expensive than their Soviet built counterparts. This Loss exchange ratio was the impetus for American forces to resurvey and retrain their pilots in programs such as TOPGUN.