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Letting last renewal number R = 1,2,3, and renewal to full term F = 0,1, I will therefore be using the following rules of thumb:
*appyear < 1981, expyear = gyear + 17
*1981 <= appyear <= 1995, expyear = gyear + 4*R +
5*F *1995 < appyear <= 2020 , expyear = appyear + 4*R + 8*F
rare cases when I don't have data on renewals for a patent filed after 1980, I will assume renewal to full term.