This dataset contains two relevant variables.  The first, gdp, is U.S. gross domestic product (GDP)
and the second, receipts, is total tax receipts of the federal governement.  The variables are
measured annually from 1960 to 1995 and measured in billions of dollars.  Since most federal tax
receipts come from sources linked to income, if GDP rises, we expect that the federal tax receipts
will increase.  These observations can be considered independent, though the GDP of one year may
prompt the government to implement fiscal or monetary policy that will affect the GDP or receipts
collected next year.