Changes in src/sas/sasgui/perspectives/fitting/media/fitting_help.rst [5005ae0:47ace50] in sasview

File:

• src/sas/sasgui/perspectives/fitting/media/fitting_help.rst (modified) (4 diffs)

src/sas/sasgui/perspectives/fitting/media/fitting_help.rst

@@ -426 +426 @@
 See :ref:`Assessing_Fit_Quality`.
 
-The objective of model-fitting is to find a *physically-plausible* model, and set
-of model parameters, that generate a theory that reproduces the experimental data
-and gives residual values as close to zero as possible.
+The objective of model-fitting is to find a *physically-plausible* model, and
+set of model parameters, that generate a theory that reproduces the experimental
+data and minimizes the values of the residuals.
 
 Change the default values of the model parameters by hand until the theory line
-starts to represent the experimental data. Then uncheck the tick boxes alongside
-all parameters *except* the 'background' and the 'scale'. Click the *Fit* button.
-SasView will optimise the values of the 'background' and 'scale' and also display
-the corresponding uncertainties on the optimised values.
-
-*NB: If no uncertainty is shown it generally means that the model is not very*
-*dependent on the corresponding parameter (or that one or more parameters are*
-*'correlated').*
-
-In the bottom left corner of the *Fit Page* is a box displaying the normalised value
-of the statistical $\chi^2$ parameter returned by the optimiser.
+starts to represent the experimental data. Then check the tick boxes alongside
+the 'background' and 'scale' parameters. Click the *Fit* button. SasView
+will optimise the values of the 'background' and 'scale' and also display the
+corresponding uncertainties on the optimised values.
+
+.. note::
+   If the uncertainty on a fitted parameter is unrealistically large, or if it
+   displays as NaN, the model is most likely a poor representation of the data,
+   the parameter in question is highly correlated with one or more of the other
+   fitted parameters, or the model is relatively insensitive to the value of
+   that particular parameter.
+
+In the bottom left corner of the *Fit Page* is a box displaying a normalised
+value of the statistical $\chi^2$ parameter (the reduced $\chi^2$,
+See :ref:`Assessing_Fit_Quality`) returned by the optimiser.
 
 Now check the box for another model parameter and click *Fit* again. Repeat this
-process until most or all parameters are checked and have been optimised. As the
-fit of the theory to the experimental data improves the value of 'chi2/Npts' will
-decrease. A good model fit should easily produce values of 'chi2/Npts' that are
-close to one, and certainly <100. See :ref:`Assessing_Fit_Quality`.
+process until all relevant parameters are checked and have been optimised. As
+the fit of the theory to the experimental data improves, the value of 'Reduced
+Chi2' will decrease. A good model fit should produce values of Reduced Chi2
+close to one, and certainly << 100. See :ref:`Assessing_Fit_Quality`.
 
 SasView has a number of different optimisers (see the section :ref:`Fitting_Options`).
@@ -462 +466 @@
 *the Data Explorer is checked (see the section* :ref:`Loading_data` *).*
 
-This mode is an extension of the :ref:`Single_Fit_Mode` that fits two or more data
-sets *to the same model* simultaneously. If necessary it is possible to constrain
-fit parameters between data sets (eg, to fix a background level, or radius, etc).
+This mode is an extension of the :ref:`Single_Fit_Mode` that allows for some
+relatively extensive constraints between fitted parameters in a single *FitPage*
+or between several *FitPage*'s (eg, to constrain all fitted parameters to be the
+same in a contrast series of *FitPages* except for the solvent sld parameter,
+constrain the length to be twice that of the radius in a single *FitPage*,
+fix the radius of the sphere in one *FitPage* to be the same as the radius of
+the cylinder in a second *FitPage*, etc).
 
 If the data to be fit are in multiple files, load each file, then select each file
@@ -501 +509 @@
 next to *Add Constraint?* in the *Fit Constraints* box.
 
+To constrain all identically named parameters to fit *simultaneously* to the
+same value across all the *Fitpages* use the *Easy Setup* drop-down buttons in
+the *Const & Simul Fit* page.
+
 *NB: You can only constrain parameters that are set to refine.*
+
+Constraints will generally be of the form
+
+  Mi Parameter1 = Mj.Parameter1
+
+however the text box after the '=' sign can be used to adjust this
+relationship; for example
+
+  Mi Parameter1 = scalar \* Mj.Parameter1
+
+A 'free-form' constraint box is also provided.
+
+Many constraints can be entered for a single fit.
 
 When ready, click the *Fit* button on the *Const & Simul Fit* page, NOT the *Fit*
 button on the individual *FitPage*'s.
 
-Simultaneous Fits without Constraints
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 The results of the model-fitting will be returned to each of the individual
 *FitPage*'s.
 
-Note that the chi2/Npts value returned is the SUM of the chi2/Npts of each fit. To
-see the chi2/Npts value for a specific *FitPage*, click the *Compute* button at the
-bottom of that *FitPage* to recalculate. Also see :ref:`Assessing_Fit_Quality`.
-
-Simultaneous Fits with Constraints
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-Use the *Easy Setup* drop-down buttons in the *Const & Simul Fit* page to set
-up constraints between *FitPage*'s.
-
-Constraints will generally be of the form
-
-  Mi Parameter1 = Mj.Parameter1
-
-however the text box after the '=' sign can be used to adjust this
-relationship; for example
-
-  Mi Parameter1 = scalar \* Mj.Parameter1
-
-A 'free-form' constraint box is also provided.
-
-Many constraints can be entered for a single fit.
-
-The results of the model-fitting will be returned to each of the individual
-*FitPage*'s.
-
-Note that the chi2/Npts value returned is the SUM of the chi2/Npts of each fit. To
-see the chi2/Npts value for a specific *FitPage*, click the *Compute* button at the
-bottom of that *FitPage* to recalculate. Also see :ref:`Assessing_Fit_Quality`.
+Note that the Reduced Chi2 value returned is the SUM of the Reduced Chi2 of
+each fit. To see the Reduced Chi2 value for a specific *FitPage*, click the 
+*Compute* button at the bottom of that *FitPage* to recalculate. Note that in
+doing so the degrees of freedom will be set to Npts.
+See :ref:`Assessing_Fit_Quality`.  Moreover in the case of constraints the
+degrees of freedom are less than one might think due to those constraints.
 
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@@ -766 +765 @@
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-.. note::  This help document was last changed by Paul Butler, 10 September
-   2017
+.*Document History*
+
+| 2017-09-10 Paul Butler
+| 2017-09-15 Steve King
+| 2018-03-05 Paul Butler