How have 25- to 34-year-olds with university degrees fared financially since 1984? To answer this question, the Pew Research Center conducted surveys in 1984, 1996, and 2009 recording monthly earnings for graduates with three different types of degrees. The incomes were converted into 2012 dollars.

a. Is there sufficient evidence to infer that incomes differed for 25- to 34-year-olds with professional or doctorate degrees between the three periods?

b. What are the required conditions?

c. Are they satisfied? Explain.