I have a dataset like this:

ID Date
10 1/2/2009
10 1/2/2009
10 1/2/2009
11 2/3/2009
11 2/3/2009
11 4/5/2009
11 4/5/2009
12 5/5/2009
12 5/5/2009
12 6/6/2009

Basically, there can be multiple dates for each. How do I group them
based on ID and and groups of different dates to create a new variable
called DateGroup?

ID Date DateGroup
10 1/2/2009 1
10 1/2/2009 1
10 1/2/2009 1
11 2/3/2009 1
11 2/3/2009 1
11 4/5/2009 2
11 4/5/2009 2
12 5/5/2009 1
12 5/5/2009 1
12 6/6/2009 2

Any help would be appreciated!!

Pauline