Standard Curves

Type of Curve

This is used to select the type of curve to be fitted. There are five general classes of curve; each has several variations.

To help with the selection of an appropriate curve you can examine example plots of each type.

Response Variate

Specifies the name of the variate containing the response (y) values.

Explanatory Variate

Specifies the name of the variate containing the explanatory (x) values.

Groups

For data classified into groups, different curves can be fitted to each group, possibly constrained by restricting some of the parameters to values common to all groups. The groups are specified using a factor.

The first model to be fitted is a single curve with common parameters over all the groups. Next the model is extended to include a different constant parameter for each group, giving a set of parallel curves, one for each group. The next model has all linear parameters, including the constant, different for each group. (The linear parameters are those, like b in the exponential curve, with respect to which the model is linear.) Finally, completely separate curves are fitted, in which all the parameters differ between groups. The list adjacent to the Groups box allows you to select between the types of regression model that you want to fit.

Graphs and further output from this analysis will display information from the final (full) model. The accumulated display setting is particularly useful, producing an accumulated analysis of variance which allows you to assess how complicated a model is really needed. The final line of the table checks whether separate nonlinear parameters are needed. If these are not required, the previous line, representing the interaction between the explanatory variate and the groups factor, can be used to assess whether separate linear parameters are required. If not, the line before that, representing the main effect of the groups factor, assesses whether different constants are needed. You can use FITCURVE in command mode to fit a particular submodel, for example:

FITCURVE [CURVE=logistic] variate + factor

fits separate constants for each group, giving the parallel curves model.

Available Data

This lists data structures appropriate to the current input field. It lists either variates for specifying the response or explanatory variate, or a factor for specifying groups. The contents will change as you move from one field to the next. Double-click on a name to copy it to the current input field; alternatively, you can type in the name directly.

Action Buttons

Run	Run the analysis.
Cancel	Close the menu without further changes.
Options	Opens a dialog where additional options and settings can be specified for the analysis.
Defaults	Set the menu settings back to the default settings. Clicking the right mouse on this button produces a shortcut menu where you can choose to set the menu using the currently stored defaults or the GenStat default settings.
Save	Opens a dialog where you can save results from the analysis.
Further output	Opens a dialog for specifying further output from the analysis and displaying residual and fitted model graphs.