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My Cubic Spline has a kink in it

Document ID: DQ010
Relevant to following products: Manta WorkOut 2.0
Document description: How To: Use Cubic Spline with non-linear data…

The nature of the Cubic Spline means that kinks may occur with particular data sets (especially non-linear);. This is because an exact curve is calculated through the data consisting of cubic segments, with continuous derivatives.
There are advanced options to handle this situation, to access these options:
  1. Edit the transform (press Edit Transforms… and select the Standard Curve Fit and press Edit… or right click on the graph under the Analysis tab and select Edit Transformation) and select the Method tab, press the Advanced… button.
  2. If the specified concentrations (X axis) are non-linear and cover a wide range, tick the Log/Anti-Log X option.
  3. If the measurements (Y axis) are logarithmically related rather than linearly, then tick the Log/Anti-Log Y option.

An example results file demonstrating this situation is available also an Excel file reproducing the situation with a 3rd party implementation of the spline. These files can be downloaded from here.

Related articles:
DQ023: Can I extrapolate with the Cubic Spline?
How to: Allow/disallow extrapolation with a Cubic Spline…

DQ024: Why are the MSE and SS of my Cubic Spline large?
How is the MSE and SS of Cubic Spline calculated?

DQ065: Which curve fit should I use?
How To: Choose a curve fit method...

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