FORECAST is predicting growth of roughly 19 per month, every month.
This increase is from a larger base every month. Divide 19 by 601 and
you get 3.16%. Divide 19 by 1242 gives you 1.53%. To grow at a
constant percentage, the amount of growth must rise. In my examples,
to get 3.16% growth on a base of 1242 one must grow by about 39.

Mark Lincoln

On Jun 12, 3:02 pm, BenS wrote:
I have 13 months worth of monthly data that I want to trend out for about 3
years on a month-by-monty basis. I am using the FORECAST function but am
puzzled by what I am seeing. I get values that visually "make sense" based
on the historical data, but when I calculate the month-over-month rate of
change I see puzzling results.

Here is the data:
Apr-06 May-06 Jun-06 Jul-06 Aug-06 Sep-06 Oct-06 Nov-06 Dec-06 Jan-07 Feb-07 Mar-07 Apr-07

329 369 390 448 442 437 451 504 552 487 574 610 511

So I am projecting out to April 2010. If you try this confirm that you get
1262 as the value for April 2010.

The thing that puzzles me is that if I chart this data it looks perfectly
linear for the forecasted values. Thus I would expect a uniform rate of
growth. But when I look at the month-over month growth rate, it steadily
declines from 3.197% in Jun 2007 to 1.546% by April 2010.

Can anyone help me understand what is going on? Is FORECAST the best way to
do what I am atttempting?

Thanks in advance for any assistance!