using GROWTH function question

[Pete Derkowski]

Hi,

I have some data taken over the course of months, and I want to predict
future months.

I'm trying to use the Growth function since it's not a linear progression,
and I can't get the function to generate what I think are meaningfull
results.

Here's a snapshot of the 'widget' data, entered into cells (including
lables) A1 - B25



Month Widgets
1 1
2 95
3 441
4 1,285
5 11,850
6 12,633
7 10,373
8 61,720
9 77,822
10 80,726
11 85,544
12 80,879
13 95,041
14 92,435
15 111,895
16 153,230
17 166,400
18 335,807
19 356,619
20 443,119
21 530,278
22 531,778
23 535,278
24 536,416



If I use the array formula =growth(B2:B25, A2:A25,A26:A30) and entering it
by selecting cells B26- B31, enter the formula, and press ctrl-shift-enter,
I get the following: (the values in cells A26-A31 were typed in and exiting
prior to formula creation - they are the months I want to predict).


25 4166515.647
26 6091791.585
27 8906704.754
28 13022341.37
29 19039743.59
30 27837684.91


Which makes no sense... the next value for month 25 will not be 4.1 million.
???

Does anyone know what I am doing wrong? I've tried TREND as well as
FORCAST (since growth didn't work, I played some) and neither generates
numbers that seem logical. ??

Pete Derkowski

 

[Jerry W. Lewis]

The GROWTH function assumes that ln(y) is linear in x. For your data,
that is only approximately true for x>8.

If you know an appropriate model for your data, you can use Excel to
estimate the parameters of that model, but Excel has no artificial
intellegence to suggest an appropriate model.

Jerry