Tally Index   <<   >>

 
   x
IBM 
AAPL
GOOG
GOOG
AAPL
MSFT
IBM 
AAPL 
AAPL
IBM 
    x
10
    3.14159
1

   ⍯ x  ⍝ {⍬⍴⍴⍵}⍯ x; group count
3 4 2 1

   {(+⌿⍵)÷⍵} 3 1 4 1 5 9
3.83333