Survival rates in extremely low birthweight infants depend on the denominator: avoiding potential for bias by specifying denominators.
Guillen U, DeMauro S, Ma L, Zupancic J, Wang E, Gafni A, Kirpalani H. Survival rates in extremely low birthweight infants depend on the denominator: avoiding potential for bias by specifying denominators. Am J Obstet Gynecol. 2011 Oct; 205(4):329.e1-7.