Pivot tables and Standard Deviation

[Guest]

I have columns of data that decrease in value that I collate using a pivot
table.

Table
Object Run Time Mass
100 0 36.1791
100 1 36.1709
100 2 36.1617
100 3 36.1527
100 4 36.1449
100 5 36.1367
100 6 36.1292
100 7 36.1209
100 8 36.1138
101 0 36.2029
101 1 36.1931
101 2 36.1837
101 3 36.1756
101 4 36.1682
101 5 36.1604
101 6 36.153
101 7 36.1455
101 8 36.1373
102 0 34.5683
102 1 34.5598
102 2 34.5515
102 3 34.5439
102 4 34.5366
102 5 34.5285
102 6 34.5215
102 7 34.5142
102 8 34.5066

After setting up the pivot table with 'Mass' as the target, I double click
the 'Count of Mass' and modify the table to my needs. ie Average, StdDev and
so forth.
Using the "PivotTable Field" ->options I selected Difference from (previous)
for in this particular case "Run Time".

Problem.
If I use the StdDev function I get negative numbers showing up in the table.
Since standard deviation is by definition the root of a square, I find the
result disturbing not to mention a little disconcerting for my research since
I have been depending on Excel for other number crunching statistics.

What is going on?

Tom

 

[Ed Ferrero]

Hi tom,

You are seeing the difference of StdDev(at RunTime t) from StdDev(at RunTime
t-1), so yes you will get negative numbers.

Ed Ferrero
http://edferrero.m6.net/