OK let me explain...

I have an excel file with 13,000 cancer cases. Each case (row) is identified
by an individual patient's unique ID number and the year/month/day of their
diagnosis. Each patient is listed in the database at least twice (at least 2
cancer diagnoses).

Right now I have the data sorted by patient ID, then by date of diagnosis. I
need to number the cases (in a new column) 1, 2, 3, etc. in the order they
present for each individual patient.

Example:

Patient ID year_diagnosis NEW COLUMN
0001 1981 1
0001 1988 2
0001 2001 3
0002 1985 1
0002 2003 2

Is there any way to automatically do this?

Any help appreciated...thanks!

JG