The "Back to Sleep" campaign resulted in a dramatic decrease in sudden infant death syndrome (SIDS) worldwide. SIDS mortality has continued to decline (in New Zealand by 63% from 1993 to 2004), but the reason for this has not been explained. A postal survey found that the proportion of infants sleeping on their back has increased substantially (from 24.4% in 1992 to 72.3% in 2005), and this could account for the 39%-48% decrease in SIDS mortality.