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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.
This analysis is available in the XLStat-Base addin for Microsoft Excel™