st: Filling missing depr. for panel

[Woong.Chung@colorado.edu]

 HEllo.
I have multi Variable Panel dataset believing
Yit=B0it+B1*X1it+B2*X2it+B3*X3it+..  The problem I need to take care is that
there are missing Y(every 4 years basis ) such that:

 ID  YR     Y    X1   X2    X3
 1   1987   1    3    4      5
 1   1988   .    4    5      7
 1   1989   .    7    3      1
 1   1990   .    8    9      4
 1   1991   .    7    5      6
 1   1992   6    7    6      10

 2  1987    3    4    7      9
 2  1988    .    9    10     14
 2  1989    .    10   8      20
 2  1990    .    7    10     14
 2  1991    .    14   12     10
 2 1992     2    24   10     16
 (note that this data is made randomly for illustrated purpose)