[R] Drop firms in unbalanced panel if not more than 5 observations in consecutive years for all variables

[Christian Schoder]

Dear R-user,

a few weeks ago I consulted the list-serve with a similar question.
However, my task changed a little but sufficiently to get lost again. So
I would appreciate any help on the following issue.

I use the plm package and work with firm-level data in a panel. I would
like to eliminate all firms that do not fulfill the requirement of
having an observation in every variable used for at least x consecutive
years. 

For illustration of the problem assume the following data set
> data
   id year  y  z
1   a 2000  1  1
2   b 2000 NA  2
3   b 2001  3  3
4   c 1999  1  1
5   c 2000  2  2
6   c 2001  4 NA
7   c 2002  5  4
8   d 1998  6  5
9   d 1999  5 NA
10  d 2000  6  6
11  d 2001  7  7
12  d 2002  3  6
where id is the index of the firm, year the index for the year, and y
and z are variables. Now, I would like to get rid of all firms with,
let's say, less than 3 consecutive years in which there are observations
for every variable. Hence, the procedure should yield
> data.reduced
   id year  y  z
1   d 1998  6  5
2   d 1999  5 NA
3   d 2000  6  6
4   d 2001  7  7
5   d 2002  3  6

Thank you very much for any help!

Cheers, Christian