The term 'prevalence' refers to the estimated population living with permanent childhood hearing loss at any given time. Estimates of the prevalence of permanent hearing loss in children under five vary according to the source of data and the criteria for defining hearing loss. Most estimates suggest a prevalence of 1 to 3 per 1000 children based on screening and/or medical records. Prevalence estimates based on parental surveys are significantly higher, as is the prevalence in neonatal intensive care units.