Index: src/sas/guiframe/documentation_window.py
===================================================================
--- src/sas/guiframe/documentation_window.py	(revision 3db44fbfe87ababed011758181e38e2806c5aac2)
+++ src/sas/guiframe/documentation_window.py	(revision f202138a6e2c8473eaa18931ba791130fca4f5c6)
Index: src/sas/perspectives/fitting/media/fitting.rst
===================================================================
--- src/sas/perspectives/fitting/media/fitting.rst	(revision e3b9001d39ae2571dd797a39a02158454cbdade1)
+++ src/sas/perspectives/fitting/media/fitting.rst	(revision b404a3decbabc5235d7b5fdffbfb0ffc19c10150)
@@ -8,4 +8,6 @@
 
    Fitting Data <fitting_help>
+
+   Assessing Fit Quality <residuals_help>
 
    Polydispersity Distributions <pd_help>
Index: src/sas/perspectives/fitting/media/fitting_help.rst
===================================================================
--- src/sas/perspectives/fitting/media/fitting_help.rst	(revision 5ac0a7a06c8fb10d07d5e589b04288e8ae73b3b4)
+++ src/sas/perspectives/fitting/media/fitting_help.rst	(revision ff42a2670776f75edbb9cf36dab0fe0c487a94e8)
@@ -351,4 +351,5 @@
 window, and a second graph window will appear displaying the residuals (the
 difference between the experimental data and the theory) at the same X-data values.
+See :ref:`Assessing_Fit_Quality`.
 
 The objective of model-fitting is to find a *physically-plausible* model, and set
@@ -373,5 +374,5 @@
 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 zero, and certainly <100.
+close to zero, and certainly <100. See :ref:`Assessing_Fit_Quality`.
 
 SasView has a number of different optimisers (see the section :ref:`Fitting_Options`).
@@ -397,5 +398,6 @@
 *Send To Fitting*. Either way, the result should be that for *n* data sets you have
 2\ *n* graphs (*n* of the data and model fit, and *n* of the resulting residuals). But
-it may be helpful to minimise the residuals plots for clarity.
+it may be helpful to minimise the residuals plots for clarity. Also see
+:ref:`Assessing_Fit_Quality`.
 
 *NB: If you need to use a customized model, you must ensure that model is available*
@@ -439,5 +441,5 @@
 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.
+bottom of that *FitPage* to recalculate. Also see :ref:`Assessing_Fit_Quality`.
 
 Simultaneous Fits with Constraints
@@ -465,5 +467,5 @@
 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.
+bottom of that *FitPage* to recalculate. Also see :ref:`Assessing_Fit_Quality`.
 
 .. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
@@ -525,5 +527,6 @@
 If you select a 'Chi2' value and click the *View Fits* button a graph of the
 residuals for that data set is displayed. Again, if you select multiple 'Chi2'
-values then all the residuals data will appear on one graph.
+values then all the residuals data will appear on one graph. Also see
+:ref:`Assessing_Fit_Quality`.
 
 Chain Fitting
Index: src/sas/perspectives/fitting/media/residuals_help.rst
===================================================================
--- src/sas/perspectives/fitting/media/residuals_help.rst	(revision 68f5fbbf51a010d6b57bb50d5c85ce7009d57c97)
+++ src/sas/perspectives/fitting/media/residuals_help.rst	(revision 68f5fbbf51a010d6b57bb50d5c85ce7009d57c97)
@@ -0,0 +1,66 @@
+.. residuals_help.rst
+
+.. |chi| unicode:: U+03C7
+
+
+.. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
+
+.. _Assessing_Fit_Quality:
+
+Assessing Fit Quality
+---------------------
+
+When performing model-fits to some experimental data it is helpful to be able to
+gauge how good an individual fit is, how it compares to a fit of the *same model*
+*to another set of data*, or how it compares to a fit of a *different model to the*
+*same data*.
+
+One way is obviously to just inspect the graph of the experimental data and to
+see how closely (or not!) the 'theory' calculation matches it. But *SasView*
+also provides two other measures of the quality of a fit:
+
+*  |chi|\  :sup:`2` (or 'Chi2'; pronounced 'chi-squared')
+*  *Residuals*
+
+.. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
+
+Chi2
+^^^^
+
+Chi2 is a statistical parameter that quantifies the differences between an observed
+data set and an expected dataset (or 'theory').
+
+*SasView* actually returns this parameter normalized to the number of data points,
+*Npts* such that
+
+  *Chi2/Npts* = { SUM[(*Y_i* - *Y_theory_i*)^2 / (*Y_error_i*)^2] } / *Npts*
+
+This differs slightly from what is sometimes called the 'reduced chi-squared'
+because it does not take into account the number of fitting parameters (to
+calculate the number of 'degrees of freedom'), but the 'normalized chi-squared'
+and the 'reduced chi-squared' are very close to each other when *Npts* >> number of
+parameters.
+
+For a good fit, *Chi2/Npts* tends to 0.
+
+*Chi2/Npts* is sometimes referred to as the 'goodness-of-fit' parameter.
+
+.. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
+
+Residuals
+^^^^^^^^^
+
+A residual is the difference between an observed value and an estimate of that
+value, such as a 'theory' calculation (whereas the difference between an observed
+value and its *true* value is its error).
+
+*SasView* calculates 'normalized residuals', *R_i*, for each data point in the
+fit:
+
+  *R_i* = (*Y_i* - *Y_theory_i*) / (*Y_err_i*)
+
+For a good fit, *R_i* ~ 0.
+
+.. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
+
+.. note::  This help document was last changed by Steve King, 08Jun2015
