我有不同年份的个人收入数据,数据框架类似于:
case.id <- c(1,1,1,1,1,1,2,2,2,2,2,2,3,3,3,3,3,3,4,4,4,4,4,4)
years <- c(1998,2000,2002,2004,2006,2008,1998,2000,2002,2004,2006,2008,
1998,2000,2002,2004,2006,2008,1998,2000,2002,2004,2006,2008)
income <- c(1100,NA,NA,NA,NA,1300,1500,1900,2000,NA,2200,NA,
NA,NA,NA,