Manual Curve Fitting and Modeling

The Curve Fit dialog allows you to adjust the chosen equation so that it comes as close as possible to your data. Each equation has a general form with a set of coefficients that you can adjust.

A tutorial covering curve fitting is available. To access it, choose Open from the File menu. Open the Experiments folder, then the Tutorials folder.

To apply a manual curve fit or model to your data, following these steps.

  1. On the graph, select the range of the graph for which you want to fit a curve. If you don't select a range, the curve will be fitted to the entire range of data displayed in the graph.
  2. Choose Model from the Analyze menu. Note: You can also apply a manual curve fit or model in the Curve Fit dialog. Click in the toolbar and select Manual.
  3. Select an equation for the curve. Common functions, such as linear, exponential, inverse, and polynomial are included in the list. The active graph will now display the curve. If you select a polynomial equation you will be able to adjust the degree. If you select the variable power and Nth inverse, you will be able to adjust the power. Note: If you do an experiment that requires an equation not found on the curve fit list, you can enter the equation by clicking Define Function.... If you save the file, the custom equation will appear in the list when you open the file again.
  4. Adjust the the coefficients and/or exponents. You can try new fits by changing either the equation or the range of data selected on the graph. The Root Mean Square Error* is adjusted and displayed on the dialog. When the fit seems appropriate, click OK. The function and the Curve Fit box, which includes the curve fit information will be displayed. To edit the properties of the Curve Fit box, double-click the box.
  5. You can adjust the parameters of a Manual fit after it has been drawn on the graph. Click a parameter name in the Curve Fit box to select it. An arrow next to the parameter name will appear. Use the up and down cursor keys to increase or decrease the value. Use the left and right cursor keys to increase or decrease the step size by which the value is changed the value. You can also click the value and type in a new value. Double-clicking the box will open a dialog where you are able to change all of the parameter values at once.
  6. If you want to exclude points from the fit, return to the page, select the point or points you want to exclude in the data table, and choose Strike Through Data Cells from the Edit menu. Then return to the Curve Fit dialog to perform a new fit. The selected data points will not be graphed nor used in the curve fit.

* The Root Mean Square Error is a measure of how far away, on average, the data points are from the fitted curve. RMSE is in the units of the y-axis.

See Also: