Transform the columns into row, and you won't need to write an expression.  
You can use a crosstab query to Uncrosstab the data.

First check to make sure your table is properly normalized.  If it is,
or if it isn't and you can't do anything about it then perhaps:

http://groups.google.com/group/comp.databases.ms-access/msg/13092869c02339dd

using StdevP instead of Max?

Two Query Example:

tblA
ID AutoNumber
Value1 Double
Value2 Double
Value3 Double
ID Fld1 Fld2 Fld3
1 3.1 5.2 7.3
2 4.7 7.4 9.3

qryASingleColumn:
SELECT ID, tblA.Value1 AS theField FROM tblA UNION SELECT ID,
tblA.Value2 AS theField FROM tblA UNION SELECT ID, tblA.Value3 AS
theField FROM tblA;

!qryASingleColumn:
ID theField
1 3.1
1 5.2
1 7.3
2 4.7
2 7.4
2 9.3

qryGetStdevP:
SELECT ID, StdevP(theField) AS StdevP FROM qryASingleColumn GROUP BY ID;

!qryGetStdevP:
ID StdevP
1 1.71464281995
2 1.8873850222

James A. Fortune
MPAPoster@FortuneJames.com