Given,
The population of the county in 2000 is 844.
The population of the county in 2009 is 856.
The population of the county in 2015 is 810.
Here, the population of 2000 and 2009 is given.
As known, the value changes linearly between two points.
When the value changes linearly between two points, then value of point between given points can be obtained by interpolation.
Hence, to predict the population of county interpolation is used.