average of Nth to Mth non null value

Hello,

I am currently working on a table that looks like this
(only with 400+ rows):

| X Y Z
---------------------
A | 2.3 4.1
B | 1.1 1.3 1.3
C | 3.0 7.8
D | 4.6 2.3 1.1
E | 3.4
---------------------

I want to calculate the average, per column, of the Nth
to Mth (ex 2nd to 3rd) non empty column-cells.
In the example above,
Avg for X is Average of 1.1 and 4.6,
Avg.for Y is Average of 3.0 and 2.3.

Would anyone know how to do that without using new,
purged tables for each column?

Thanks for your input!!
Steve