INTERPOLATE (Pro.)

X
is the x-component of the input data table (a one dimensional array of cells (e.g. rows or columns)).

Y
is the y-component (i.e. function) of the input data table (a one dimensional array of cells (e.g. rows or columns)).

target
is the desired x-value(s) to interpolate for (a single value or a one dimensional array of cells (e.g. rows or columns)).

Method
is the interpolation method (1=Forward Flat, 2=Backward Flat, 3=Linear, 4=Cubic Spline).

extrapolate
sets whether or not to allow extrapolation (1=Yes, 0=No). If missing, the default is to not allow extrapolation.

1. The X and Y array sizes must be identical.
2. The X-array and Y-array both consist of numerical values. Dates in Excel are internally represented by numbers.
3. The values in the X-array can be unsorted and may have duplicate values.
4. In the case where X has duplicate values, INTERPOLATE will replace those duplicate values with a single entry, setting the corresponding y-value equal to the average.
5. The X and/or Y arrays may have missing values (#N/A). In this case, INTERPOLATE will remove those entries.
6. For cubic spline interpolation, we construct a set of natural cubic splines that are twice continuously differentiable functions to yield the least oscillation about the function f which is interpolated.

Original Data

Example 1: Flat Forward Interpolation

Example 2: Flat Backward Interpolation

Example 3: Linear Interpolation

Example 4: Cubic Spline Interpolation

• Kincaid, David; Ward Cheney (2002). Numerical Analysis (3rd edition). Brooks/Cole. ISBN 0-534-38905-8. Chapter 6.
• Ahlberg, Nielson, and Walsh, The Theory of Splines and Their Applications, 1967.