Linear extrapolation

Dear who can help this out;

I have state level socioeconomic data set from census, income, race ratio, marriage ratio, education level etc. It's in each 10 year period (1980, 1990, 2000). I want to linearly interpolate each variable values for the years in between (values for 1981~1989, 1991~1999).

Data structure looks like this

State year income race ratio marriage ratio education level
Alabama 1980 10000 0.40 0.52 0.34
Alabama 1990 12000 0.44 0.50 0.32
Alabama 2000 13000 0.48 0.58 0.39
California 1980 15000 0.20 0.63 0.42
California 1990 16000 0.25 0.66 0.43
California 2000 16500 0.30 0.69 0.47

and so on...

Any idea on this particular procedure with this set of data would be appreciated!

Thank you,
MB