Marginal Analysis in Economics 

Standard

http://www.economicshelp.org/blog/2391/economics/marginal-analysis-in-economics/

Study.com (download the transcript)

http://study.com/academy/lesson/marginal-analysis-in-economics-definition-formula-examples.html#transcriptHeader
Investopedia
http://www.investopedia.com/terms/m/marginal-analysis.asp