Trend function question

[jennifer1970]

I was asked to use Excel to extrapolate the growth of used capacity for
drive size on a server. Not really knowing too much about statistical
functions, I thought I'd try to educate myself.

I found the Trend function in Excel and it seemed to do what I want.
But then I was reading about the Growth Trend - where you select a
number of cells and then drag the fill handle it it just fills in
numbers for you without a specific function.

I tried both just to see what they did and I saw that the Growth Trend
produced higher estimates than the Trend function the farther out the
estimates went.

My question is which one do I trust? What is the difference between
the two?

Thanks,
Jennifer

Results from Trend function:
Date Usage
2/2/06 36537.02
2/3/06 36542.44
2/6/06 34956.7
2/15/06 38107.00
=============
Function starts here
2/16/06 37821.75941
2/17/06 38080.03568
2/18/06 38599.78576
2/19/06 38586.30352
2/20/06 38975.31669
2/21/06 39228.4507
2/22/06 39416.2162
2/23/06 39737.28979
2/24/06 39957.73951
2/25/06 40212.15911
2/26/06 40482.92078
2/27/06 40720.35541
2/28/06 40982.94612


Results from Growth Trend Dragging:
2/2/06 36537.02
2/3/06 36542.44
2/6/06 34956.70
2/15/06 38107.00
=============
Dragging starts here
2/16/06 37316.84
2/17/06 37629.26
2/18/06 37941.68
2/19/06 38254.10
2/20/06 38566.52
2/21/06 38878.94
2/22/06 39191.36
2/23/06 39503.78
2/24/06 39816.20
2/25/06 40128.62
2/26/06 40441.04
2/27/06 40753.46
2/28/06 41065.88

 

[Bill Martin]

According to the Excel help file:

TREND Returns values along a linear trend. Fits a straight line...
GROWTH Calculates predicted exponential growth....

Which one you use depends on some knowledge of the nature of the increase you're
trying to model. Is your process intrinsically linear or exponential?

Bill

 

[Guest]

Excel's help does not do TREND justice. TREND returns values along a model
that is linear in the unknowns (multiple linear regression, polynomials,
etc.). FORECAST is the function that is limited to a straight line.

Jerry