In statistics, interpolation refers to the process of estimating a value that exists within two points on a line or curve. Linear interpolation is a relatively simple process, because plotting the data on a graph would allow one to visually locate the projected value. Interpolation on a curve requires utilizing the following equation, using the coordinates of the two points:
The units in this equation correspond to the coordinate points of each point used to extrapolate, with (x1, y1) representing one point and (x2, y2) representing the other; the coordinates including the interpolated value are (x, y). This would be the formula utilized to interpolate with frequency distribution curves. Interpolating does not require a different formula if one axis is measured in percentages.