I'm attempting to generate an exponential regression equation using Excel 2010. I plan to use the equation to predict the future condition of roads.
I've mocked up a simple chart in Excel using the data below.
AGE CONDITION
0 10
1 10
2 10
3 10
4 10
5 10
6 9
7 9
8 9
9 9
10 8
11 8
12 7
13 7
14 6
15 6
16 5
17 4
18 3
19 2
20 1
The orange line is based on the data. The black line is a regression trendline that has been auto-generated by Excel.
Problem:
It's clear to me just by looking at the graph that the exponential trendline does not match the data. In comparison, all of the other types of trendlines that Excel produces match the data fairly well.
For example, this polynomial trendline seems to fit the data like a glove:
Why does the exponential trendline not match the data?