A straight line is just a curve that never bends!
Estimating would be so much more straightforward if our estimating or forecasting technique was, well, straight forward . . . or backward, i.e. just a question of projecting values along a straight line. That way we only need to concern ourselves with one input variable and two constants (an intercept and a slope or gradient) in order to get an output variable:
Output = slope × Input + Intercept
Unfortunately, in reality many estimating relationships are not linear. However, there are some instances (especially where we need to interpolate rather than extrapolate) a linear approximation may be adequate for the purposes of estimating. As George ...