proc freq doesn

[deleted_user]

there're many teachers working for different department,most of them will teach in more than one department.count frequency of teachers for each year and each school.
sample:
department year teacher
econ 2002 Samon
econ 2003 Samon
manag 2003 Stephen
manag 2003 Sissi
Chem 2002 Sissi
Chem 2003 Samon
Math 2003 Stephen
Math 2004 Laura
Eng 2002 Laura
Eng 2003 Stephen