Curve Fitting

The curve fitting tool adjusts 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. A new column containing calculated y-values may be added to your data table if desired.

Curve Fits can be:

Automatic: The program will find the best values for the coeffecients to fit the curve to the data whenever the user presses the "Try Fit" button. If the program is having a difficult time fitting the data, you can enter some known starting values for the coeffecients, or you can press "Try Fit" more than once. (Note that if you enter coeffecient values directly, the dialog switches to Manual -- if you're giving start values for an automatic fit, you must check the Automatic radio button again)

Manual: The user types in all the coeffecient values and the program does not do any automatic fitting.

On a graph window, 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. (If you are attempting to do a curve fit on grouped graphs, make certain to select the specific graph you want the fit performed on.)

Select Curve Fit from the Analyze menu or click on the Toolbar.

Select an equation for the curve. Common functions, such as linear, exponential, inverse, and polynomial are included in the list. After you have selected a polynomial equation you will be able to adjust the degree. After you have selected the variable power and Nth inverse, you will be able to adjust the power.

Choose Manual (to experiment with these coefficients and/or exponents) or Automatic under Fit Type.

The upper portion of the screen will now contain a graph displaying the data and the best fit curve. A box containing information about the curve fitting process will appear at the bottom of the screen. The coefficients and/or exponents calculated will be presented along with the root mean square error. If the fit seems appropriate, click on the OK button. The main screen will appear with the data and fitted curve along with a helper object that displays curve fitting information. To edit the object's properties, double-click the box to summon the Helper Options dialog box.

You can try new fits by changing either the equation or the range of data selected on the graph, and then clicking the Try Fit button.

Tip: You can click on Try Fit more than once. Each time you click Try Fit, the algorithm starts from the current values of the coefficients and iterates closer to a solution.

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 select 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.