ranges of rows in other columns

[Austin Frank <austin.frank@gmail.com>]

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Hello all--

I'm looking for help in setting up a calculation in a table in org mode.
 I'd like to keep a running average of my past three days worth of
measurements.  Something like

|------------------+-----------------+-----------------|
| Date             | TV Time (hours) | Running Average |
|------------------+-----------------+-----------------|
| <2006-06-24 Sat> |               1 |                 |
| <2006-06-25 Sun> |             1.5 |                 |
| <2006-06-26 Mon> |              .5 |               1 |
| <2006-06-27 Tue> |               3 |            1.66 |
| <2006-06-28 Wed> |             2.5 |               2 |
|------------------+-----------------+-----------------|

I just did the numbers for running average in my head-- I'm hoping for
help with a formula that will get the mean for the last three rows of
the `TV Time' column.  I tried using $3=vsum($2&3-1), but this didn't
work.

Is there a syntax for accessing row ranges of other columns for a
calculation?  If not, can anyone suggest an elisp formula that will
accomplish this?

If a helpful soul is feeling especially generous, is there also a way to
set this table up that will calculate the new running average every time
I enter a new date's measurement?  Extra credit if you use Carsten's
constants.el to provide units in the measurment column and the
calculated column ;)

Thanks for any help,
/au
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