CurveExpert Basic Documentation

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Appendix: Built-In Nonlinear Regression Models

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CurveExpert Options

The Options dialog allows you to set important parameters that CurveExpert will use during operation. Two separate sets of user preferences are maintained for the single and double precision versions (registered users only).

If an invalid entry has been entered in any of the tabbed pages, then the offending entry will be selected, and the computers speaker will sound.

Preferences Dialog

Application Options

The application options tabbed page sets user preferences that control the overall behavior of CurveExpert in the Windows environment.

Data Files

  • Data File Location: The directory in which you store your data files can be input in this edit field. For convenience, each time CurveExpert is started, the directory will be automatically changed to this working directory. By default, this directory is set to be the CurveExpert home directory.
  • Default Data File Extension: If a file extension is not supplied in the File|Open or File|Save dialog box, then this extension is automatically added. Default: DAT
  • Automatic Data Sorting: Selecting this checkbox will automatically sort any data file as it is read into CurveExpert. Default: Off
  • Always show file import dialog: Selecting this checkbox will cause the file import dialog to always appear when a file is selected to read into CurveExpert. This dialog will give access to all of CurveExpert features for parsing data files. When this checkbox is cleared, CurveExpert will attempt to intelligently read the data file based on the number of columns that it detects.

Graph Rendering

The graph rendering radio buttons may be set to Slow, Medium, or Fast. Each setting has advantages and disadvantages.

  • The “Slow” setting requires no extra memory, but all graphs are drawn from scratch when their window is resized or uncovered.
  • The “Medium” setting requires a small amount of extra memory and behaves like the “Slow” setting, with the exception that any user-model graphs will be drawn using “Fast” rendering. See below; this is the default.
  • The “Fast” setting requires a large amount of extra memory, because the contents of every graph window are cached for quick redraws. The graph is only drawn the first time it is opened, and each time it is resized. Any other graph viewing will take a minimal amount of time to redraw, since the image is stored in memory. This option is most useful for users who have extremely large data sets that take a long time to draw.


  • Enable Background Processing: When CurveExpert is iterating, it can take complete control over Windows (faster), or do its processing in the background (slower). Checking this checkbox will force CurveExpert to do its calculations in the background. This is recommended, since certain windows processes need attention often. On the other hand, if CurveExpert is the only application running, then the best performance will be obtained by leaving this box unchecked. Default: Enabled
  • No graphs, only information: CurveExpert gives you the option of viewing the graph first, or the fit information first. If this checkbox is selected, the model information (coefficients, etc.) will be shown directly, instead of a graph. Default: Disabled
  • 3D Dialog Effects: Attractive (in the opinion of the author) gray dialogs with 3D effects are the default in CurveExpert. If you prefer the white dialogs with no 3D effects, deselect this checkbox. Note that this options is largely obsolete; as all versions of Windows later than Windows 95 have 3D dialog effects by default. Default: Enabled.
  • Save Main Window Position: CurveExpert saves the position of the main window when the application is exited, so that when it is opened again, it assumes the same position. If this behavior is undesirable, deselect this checkbox. Default: Enabled.

Setting the number of Undo Levels

  • Number of Undo Levels: CurveExpert implements a multilevel undo system for the built-in spreadsheet. The number of actions that can be undone can be set in this edit field. The number of levels of undo must range from 1 to 99. If your system has low amounts of memory, then a low number of undo levels will be conserve this memory. Default: 20.

Regression Options

The regression options tabbed page sets user preferences that control the regression and mathematical operation of CurveExpert.

Linear Regression

  • Maximum Polynomial Degree: Enter the maximum degree of a polynomial allowed by CurveExpert in this field. This setting determines the number of polynomials examined by CurveFinder in a search sweep, and it also prevents choosing a wildly oscillating polynomial by accident. This number may not exceed 19. Default: 10
  • Default Polynomial Degree: Enter the default degree of polynomials used in CurveExpert in this field. The default polynomial degree is the polynomial that will be calculated by the Calculate Groups command, and the degree that will appear by default when you select a polynomial from the Apply Fit menu. Default: 4

Nonlinear Regression

  • Regression Tolerance: Enter the desired tolerance for nonlinear regression convergence in this field. The nonlinear algorithm uses this number to determine whether or not it has converged on the correct model parameters. Mathematically speaking, the difference between the standard errors in two consecutive iterations must be smaller than this tolerance for the computation to terminate. The smaller the tolerance, the more accurate the parameters will be, but the algorithm will take much more time to converge. Default: 1e-6
  • Diverging Iterations: Enter the number of iterations that the standard error (mentioned above) must increase for the algorithm to declare divergence. Default: 10
  • Maximum Iterations: Enter the maximum number of iterations for the nonlinear algorithm to perform before it is forced to terminate. Default: 100

General Regression

  • Weighting: CurveExpert can either perform no weighting for each point, or weight each point by 1/y^2 as the nonlinear regression progresses. Using no weighting is most appropriate when no known relationship exists between the variances of the data around the fitted curve and the y values of the points. However, it is often the case in experiments that it is known that the uncertainty is a certain percentage of the y value; in this case, weighting the points by 1/y^2 can prevent the larger scatter at higher y from adversely affecting the regression model. Default: No Weighting
  • Automatic Initial Guesses: Enabling this checkbox will let CurveExpert generate initial guesses for built-in nonlinear regressions for you. This option, in effect, enables AutoGuess upon startup. Default: Enabled
  • Quick Derivatives: The partial derivatives of the model with respect with each of its parameters is required in the Levenberg-Marquardt algorithm for nonlinear regression. CurveExpert uses central differencing to approximate these partial derivatives; however, more precise approximations can be accomplished using Richardsons extrapolation. Checking the “Quick Derivatives” checkbox uses simple central differencing; leaving this checkbox unselected forces CurveExpert to use Richarsons extrapolation for the numerical derivatives. The latter is more accurate, but much slower than using central differences. Default: Enabled

Printing Options

The printing preferences tabbed page sets user preferences that control printing settings and margins that CurveExpert will use for graph hardcopies and data tables.

  • Current Printer Info: This section shows all information for the currently selected printer. The name, driver version, orientation, paper size, and resolution are shown.
  • Default Print Type: The default print type determines whether the “Black and White” or “Color” printing selection will be automatically selected when a graph is printed. Of course, the user may change the selection at print-time. This option is included for the convenience of color printer users. Default: Black and White.
  • Print Margins: The top, left, bottom, and right margins (in inches) can be set in each of these edit fields. Note that no margin can be less than 0 inches, and no corresponding margins can add to higher than the paper extent plus the unprintable margin.
  • Print Setup: Press the Print Setup button to change the current printer and/or the printer configuration.