Re: growth rates not totalling
- From: Richard Ulrich <Rich.Ulrich@xxxxxxxxxxx>
- Date: Wed, 20 Sep 2006 20:57:56 -0400
On 20 Sep 2006 10:26:38 -0700, "Native" <native_texan_03@xxxxxxxxx>
wrote:
New to this group so any help appreciated.
Here is my issue. I'm performing a growth rate calculation and then
applying that growth to project a full-year estimate. Everything looks
good. However, if I try to add up the parts, I get a different answer
than if I do the calculation against the whole. Any thoughts?
You can't expect inhomogeneous growth rates to
give the same results when aggregated from different
base Ns.
Example Data Below:
YTD = year to date
Neighborhood A
Year 1 105YTD 245Full-Year
Year 2 120 ?
YTD Growth = (120/105)-1 = 14%
Applying the 14% to 245 I get 280 for a full-year estimate.
Neighborhood B
Year 1 73YTD 197Full-Year
Year 2 112 ?
YTD Growth (112/73)-1=53%
Applying the 53% I get a full-year estimate of 302
TOTAL
Year 1 178YTD 442
Year 2 232 ?
YTD Growth (232/178)-1=30%
Applying the 30% to 442 I get 575.
My issue is 280 + 302 does not equal 575. Any thoughts?
I have trouble figuring where some of your numbers
come from, but I think this is an illustration of what you
have, with more extreme numbers.
Two growth curves -
1 - 10 - 100
25 - 26 - 27
Aggregated -
26 - 36 - 50
Hope this helps.
--
Rich Ulrich, wpilib@xxxxxxxx
http://www.pitt.edu/~wpilib/index.html
.
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