# Cubic splines

Cubic splines are frequently used for interpolation. Run cubic splines in Excel using the XLSTAT add-on statistical software.

## What is a cubic spline?

A cubic spline is defined as a piecewise function of polynomials of degree 3. Cubic splines are used in interpolation problems where they are preferred to usual polynomial interpolation methods. The reason being that they allow a compromise between the smoothness of the curve and the degree of the polynomial.

## Use of the cubic splines in XLSTAT

Cubic splines are defined from a set of nodes. In XLSTAT, three options are offered:

Use data as nodes

Define a number of nodes equally distributed.

Choose a set of nodes

## Results for the cubic splines in XLSTAT

**Coefficients of the polynomials:**This table displays the coefficients of polynomials of degree 3 of the cubic spline.**Prédictions and residuals:**For each observation, the value of the dependent variable, the predictions, the residuals and the standardized residuals are displayed.**Spline curve:**The cubic spline and the selected nodes are displayed.

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