Absolute pitch (AP) is defined as the ability to identify the pitch of a musical tone or to produce a musical tone at a given pitch without the use of an external reference pitch.[23] Researchers estimate the occurrence of AP to be 1 in 10,000 people.[24] The extent to which this ability is innate or learned is debated, with evidence for both a genetic basis and for a "critical period" in which the ability can be learned, especially in conjunction with early musical training.[25][26]