Steel, Daniel P. "Social Mechanisms and Process Tracing." Across the Boundaries: Extrapolation in Biology and Social Science. : Oxford University Press, 2007-10-01. Oxford Scholarship Online. 2008-01-01. Date Accessed 20 May. 2013 <http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195331448.001.0001/acprof-9780195331448-chapter-9>.
Steel, D.(2007-10-01). Social Mechanisms and Process Tracing. In Across the Boundaries: Extrapolation in Biology and Social Science. : Oxford University Press. Retrieved 20 May. 2013, from http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195331448.001.0001/acprof-9780195331448-chapter-9.
Steel, Daniel P. "Social Mechanisms and Process Tracing." In Across the Boundaries: Extrapolation in Biology and Social Science. : Oxford University Press, 2007-10-01. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195331448.001.0001/acprof-9780195331448-chapter-9.
Steel DP. Social Mechanisms and Process Tracing. In: Steel D. Across the Boundaries: Extrapolation in Biology and Social Science. Oxford University Press; 2007. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195331448.001.0001/acprof-9780195331448-chapter-9. Accessed May 20, 2013.