Bostrom, Nick and Ćirković, Milan M. 2003. “The Doomsday Argument and the Self-Indication Assumption. Reply to Olum (2002).”The Philosophical Quarterly 53(210): 83–91.
Further References
Olum, Ken D. 2002. “The Doomsday Argument and the Number of Possible Observers.”The Philosophical Quarterly 52(207): 164–184.