Help: Oracle MEAN Function

     Howdy All:

     My problem is as follows:

     If

         table RECORD (
           office             varchar2(3),
           type               varchar2(3),
           type_amt           number(9)
         )

     Where

          field 'type' is either 'ASP', 'LBR', 'MAT'
          and has a value ('type_amt') associated
          with it.

     How would I find

         The MEAN for 

                  each office
                     each type

         if MEAN is defined as 

            ( MAX(type_amt) + MIN(type_amt) ) / 2

     I looked thru the manual and there are only STDDEV, AVG,
     and a few other statistical functions.

     Do I have to write my own PL/SQL function to do this or
     has something similar been written already that I could
     study ?

     I have 2,000,000 records, 200+ offices.

     How do I approach this so my code isn't sluggish
     or a resource hog ? I dread the thought of 
     creating a PL/SQL array of 200 'offices'.

     Does Oracle have a set of 'magic' statistical function bullets
     somewhere ?


     Thanks in Advance,

     Eisen