Read the September 2016 article about contract or adjunct faculty in American higher education:
As an adjunct, Wangerin is employed on a casual basis and earns somewhere between half and one-third of what a tenure-track professor would make for teaching the same courses. That is significant, because non-tenure track teaching staff – commonly referred to as adjuncts and contingent faculty – now make up approximately 70% of all teaching staff in American higher education. This means that roughly three out of every four courses a student takes are taught by someone without job security who is working on minimal pay.
Go to the Salon.com article at http://www.salon.com/2016/09/24/life-in-academic-poverty-as-an-underpaid-university-teacher-they-just-do-not-want-to-pay_partner/