Frequently Asked Questions
Here are the answers to some common questions about SasView:
What is SasView?
 SasView is software for the analysis of SmallAngle Scattering (SAS) data.
 It fits analytic functions describing different types of material microstructure to experimental data in order to determine the shape, size and degree of ordering.
 SasView also includes tools for calculating scattering length densities, slit sizes, resolution, fringe thicknesses/dspacings, the (Porod) invariant ('total scattering'), and distance distribution functions.
What platforms does SasView run on?
 Windows
 SasView 2.x: 32bit.
 SasView 3.x, 4.0.x and 4.1.x: 32bit & 64bit.
 SasView 4.2.x and later: 64bit.
 Mac OS
 SasView 3.x and earlier: 32bit OSX (10.6, Snowleopard).
 SasView 4.x and later: 64bit OSX (10.9, Mavericks or later).
 Linux
 We do not currently support a Linux distribution.
 SasView is being built under Ubuntu on the development servers but is mostly aspirational at present due to limited developer resources. In fact the eggs produced do not currently appear to be usable. We hope to eventually be able to distribute on Linux and welcome anyone interested in helping with that project to join us.
 In the meantime you are welcome to try building SasView from source available on the release pages: https://github.com/SasView/sasview/releases and executing 'python run.py'. Please note, you will also need to download our 'sasmodels' repository from https://github.com/SasView/sasmodels before you try this. Some people have reported success with this approach.
 There is a thirdparty project developing SasView on Debian
Do I need to install Python/C++ or any compilers before I install SasView?
 If you intend to use SasView on a Windows computer, then No: Our packaged installer ships with everything you need to run SasView.
 If you intend to use SasView on a Mac OS computer, you do not need to install Python/C++ but you do need to preinstall the Xcode command line tools as SasView makes use of the C compiler.
Can SasView make use of my GPU(s)?
 GPU support is only available in SasView 4.x and later.
 If a GPU is available then SasView will attempt to use it by default if a suitable driver is also installed. See the SasView Help documentation for further information.
Can I stop SasView trying to use my GPU(s)?
 Yes. Create a system environment variable called SAS_OPENCL and give it the value 'None'.
Can I use SasView to analyse SANS/USANS data?
Can I use SasView to analyse SAXS/USAXS data?
 Yes, especially if your intensity data is in absolute units (/cm).
Can SasView be used in a commercial environment?
 Yes, provided you accept the terms of our licence. There is no licence fee.
What format should my data be in?
1D data (I(Q) vs Q)
 Files with 2 to 4 columns of numbers in the following order: Q, I(Q), (dI(Q), dQ(Q)), where dQ(Q) is the instrumental resolution in Q and assumed to have originated from pinhole geometry. Numbers can be separated by spaces or commas.
 SasView recognises the following file extensions:
 If using CSV output from, for example, a spreadsheet, ensure that it is not using commas as delimiters for thousands.
 For a description of the NIST 1D format click here.
 For a description of the ISIS 1D format click here.
 NB: SasView does not at present load data where the Q and I(Q) data are in separate files. But this is coming in version 4.1!

2D data (I(Qx,Qy) vs Qx & Qy)
 Files in the NIST 2D format with the extension .ASC or .DAT
 Most of the header lines can be removed except the last line, and only the first three columns (Qx, Qy, and I(Qx,Qy)) are actually required.
 For a description of the NIST 2D format click here.
 Files in the NXcanSAS format with the extension .h5
What units should my data be in?
Q data
 SasView assumes that Q data is in units of /Angstroms. If your Q data is in units of /nm (or anything else) you will get incorrect parameters from your model fits. However if your data is in canSAS format and the Q unit is specified as /nm  ie, the XML specifies Q unit="1/nm"  SasView will internally convert the Q data to Å.
I(Q) data
 SasView makes no assumptions about the units of I(Q) data, but your model fits will only return meaningful scale parameters (ie, volume fractions) if the I(Q) data is in absolute units (/cm) and on an absolute scale (ie, the I(Q) values have been calibrated against a standard).
Does SasView fit data in its I vs Q representation, or as Log I vs Log Q?
 SasView assumes that the data values it loads are not Log values. But the default behaviour of the program is to display the data and fit as Log I vs Log Q.
 If you want to change how the data and fit are displayed, rightclick on the plot and choose Change Scale.
 NB: when using log representation, SasView applies Log10 and not Loge.
 However, SasView has no way of knowing if the data it has loaded is actually I vs Q. It will happily load and fit, for example, Reflectivity vs Q, but simply label the Yaxis as Intensity.
Many models have a scale parameter; what is this?
 If your I(Q) data is in absolute units (/cm) and on an absolute scale (ie, the I(Q) values have been calibrated against a standard) the scale parameter will be equal to the volume fraction. In all other instances the scale factor will be a number proportional to the volume fraction. Some models have both volume fraction and scale parameters. In these models the scale is just a multiplying factor (equal to 1 if the data is in absolute units on an absolute scale).
 We are aware that some of the model descriptions in the 'help file' are unclear about the meaning of scale and are working to improve the documentation.
How can I put my data on an absolute intensity scale?
 By comparing the intensity of scattering from your samples with that from a standard sample measured under identical conditions with the same instrument geometry. In most instances it will be a simple linear scaling, provided the sample and standard transmissions (transmission = 1  absorbtion) have been accounted for during the data reduction. Consult your beamline scientist/local contact.
Why do I get nonsensical parameter values if I try to simultaneously modelfit the scale factor and the scattering length densities?
 Because these parameters are correlated. You must either fix the scale or the scattering length densities (at least, until your fit has almost converged).
What value should I use to specify polydispersity?
 This will depend on the actual type of polydispersity function you choose, but will be related to some ratio of the 'width' of the size distribution and the mean value of the parameter that the polydispersity is being enabled for (eg, a particle radius). For example, for a Schulz polydispersity function the PD parameter in SasView is standard_deviation/average_value. In most (meaningful) instances 0 <= PD < 1.
Is the Schulz(Zimm) polydispersity function used in SasView a numberaverage or a weightaverage distribution?
 SasView assumes the underlying polydispersity function is a numberdistribution and therefore the reported mean from that distribution is a numberaverage parameter. But SasView does properly weight the function by volume when fitting to the scattering data as is required to fit data in absolute units.
 However, it is also clear that the literature has conflicting views about what the correct form of a numberaverage Schulz distribution should be. In essence there are two different distributions which are being called the Schulz distribution. These are unfortunately related through mass. SasView, like several other wellknown and wellused modelfitting packages, uses the form of the distribution given in the M. Kotlarchyk & SH. Chen paper (J. Chem. Phys. 79 (1983) 24612469) which specifically identifies it as a numberaverage distribution. This is also the same Schulz numberaverage distribution given by Aragon & Pecora (J. Chem. Phys. 1976) and Bartlett and Ottewill (J. Chem. Phys. 1992), among others. On the other hand, the earlier papers using this distribution were specifically applied to degrees of polymerization and were clear that they viewed the equivalent formula to be a weightaverage distribution. It now seems that at least Welch & Bloomfield (J. Pol. Sci. 1973) and Huber (2012) use the same equation as Aragon & Pecora's numberaverage formula but define it as a weightaverage formula and then derive a new numberaverage formula from this.
 (February 2015) After further investigation we have concluded that our implementation of the Schulz polydispersity function is sound. Our definition of the z parameter, describing the width of the distribution, has exactly the same mathematical form whether defined as a numberaverage of a numberaverage distribution or as a weightaverage of a weightaverage distribution. But in some papers z is defined as z+1 which clearly would give a different mathematical behaviour.
What about the other polydispersity functions used in SasView?
 These are implemented as numberdistributions also.
How do I load an image file as data?
 Use 'Tool' > 'Image Viewer' and **not** 'File' > 'Load Data File(s)' or the 'Load Data' button on the Data Explorer.
 SasView will load .BMP, .GIF, .JPG, .PNG and .TIF formats.
Where do the model parameter uncertainties come from?
 How the parameter uncertainties (commonly, but wrongly, called the parameter errors) are estimated depends on the fitting algorithm used. One estimate of uncertainty is to use the square root of the diagonal of the covariance matrix. This can give a good estimate if the uncertainty is well behaved, with no significant correlation between the parameters. The covariance matrix is calculated as part of the commonly used LevenbergMarquardt optimizer, which is provided by the Python SciPy library). Otherwise the covariance matrix is estimated using the numerical derivative at the best point. If you really want to know more, check out this handy tutorial by Brian Toby at APS. The DREAM optimizer uses a Monte Carlo approach which provides a more realistic estimate of the uncertainties for the case when uncertainties are not well behaved.
Why do I get different parameter uncertainties using the Simple and Complex fitting engines?
 This applies to SasView 3.0.0 and earlier.
 To a first approximation, the two fitting engines should return similar, but not identical, parameter uncertainties. The Simple engine uses the Python SciPy library which computes the Jacobian using the forward difference formula and evaluates the covariance matrix by QR decomposition. The Complex engine computes the Jacobian using the centre point formula and evaluates the covariance matrix by singular value decomposition (SV). Both methods will also be affected by the accuracy of the numerical differentiation procedures and the precision of the floating point computations.
 (July 2014) We have discovered an inconsistency in the way that the Simple and Complex fitting engines define the reduced chisquare in SasView 3.0.0 and earlier. The consequence of this is that the Complex engine is not performing as many iterations as it should and is thus returning parameter uncertainties that are perhaps a factor 5 to 10 larger than expected. A solution is under consideration.
Why do I sometimes get different 'goodnessoffit' values when pressing Enter or Compute compared to pressing Fit?
 The SasView FitPage displays a 'normalized chisquare' computed as chisquare divided by the number of points. But the fitting engines compute chisquare divided by the number of points minus the number of fit parameters. So the more parameters you fit, the worse the apparent goodnessoffit. Conversely, if you just press Enter or Compute the effective number of parameters is zero so the goodnessoffit may appear to go down.
Why have I got several instances of the same model category (eg, 'Shape Independent' and 'ShapeIndependent')?
 This is most likely because you have previously installed earlier versions of SasView and the category file needs rebuilding.
 If open, close SasView. Then go to your profile folder (eg, on Windows: C:\Users\user_name) and open the .sasview folder. Delete the file categories.json. Run SasView again.
Why is SasView not loading the model parameters from some of my existing .fitv (Save Fit) files?
 As the number of models implemented in SasView increased it became necessary to add additional categorisation. At the same time we gave the user the ability to customise the categorisation (the Modify Category button). Unfortunately this does mean that the categorisation of some models will have changed between earlier versions of SasView (when it was still called SansView) and recent releases. When SasView cannot determine the model categorisation from a .fitv file it attempts to default to the first model in the first category. There are two workarounds to this issue:
 1) Open the .fitv file in a text editor and scroll down to the section headed Attributes. Above the entry formfactorcombobox create a new entry categorycombobox categorycombobox="type", where type is the category that the model was originally in (current categories are: ShapeIndependent, Shapes, Structure Factor, Uncategorized, Custom Models). Make sure you enclose your new entry in angle brackets just like the existing entries!
 2) Use the Modify Category button in the recent version of SasView to move the model you want to use to the category it used to be in.
Why is Save Analysis (File menu) greyed out after I fit a model?
 At present Save Analysis gets greyed out if you click on a graph window after fitting a model. We agree this is not very sensible behaviour and will rectify it in a future bug release. In the meantime, use the 'Save' icon in the graphical toolbar to generate .fitv files.
Why doesn't Check for Updates work?
 Check for Updates compares the version (not build) number of your copy of SasView with that in the code repository. If SasView cannot access the remote server the message 'Cannot connect to the application server' is displayed in the status bar.
 Prior to version 3.1.1., the most common reason for Check for Updates failing was that your internet connection used a proxy server and your operating system had not been fully configured to use that proxy server. It is entirely possible to be able to use a web browser to surf the internet but find that SasView cannot read the current version number from the code repository! We believe this is fixed in 3.1.1. How you remedy this if you have a lower version (and don't want to update) depends on your operating system:
Windows 7:
 Warning! The following procedure involves changing system settings. If you are not confident about doing so, consult your local IT Helpdesk.
 If you are not sure if your internet connect uses a proxy server, click on the Start button, select 'Network and Internet' and then 'Internet Options'. Now click the 'Connections' tab and then the button 'LAN settings'.
 Click on the Start button. In the 'search programs and files' box type 'regedit' and press {return}.
 In the Registry Editor, navigate to the registry key (be careful not to find find a similar key by mistake) HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings. Look for a setting named 'ProxyServer'. If it exists, ensure it both points to your proxy server and has a port number appended. If it is missing create this setting as follows:
 Edit > New > String Value. Rightclick on 'New Value' and rename it to 'ProxyServer'. Rightclick again, choose 'Modify', and in the 'Value data' box type the address and port number of your proxy server (eg, http://proxy.uni.edu:port).
 If your internet connection also uses a proxy autoconfiguration (or .pac) file (see the instructions above for how to check), also check for the presence of a setting 'AutoConfigURL'. Again, if this exists, ensure it points at the right address and file. If it is missing, create this setting (eg, http://proxy.uni.edu/configproxy.pac) in the same manner as above.
 Note that the value (0 or 1) of the REG_DWORD called 'ProxyEnable' appears to be irrelevant!
 Close the Registry Editor.
 Click on the Start button. In the 'search programs and files' box type 'environment variables' and press {return}.
 In the Environment Variables panel, check the User variables (not System variables) for HTTP_PROXY and HTTPS_PROXY. If they exist, ensure that both specify your username, proxy server and port. If they are missing, create these variables as follows:
 Click 'New'. Enter HTTP_PROXY as the 'Variable name'. Enter a 'Variable value' of the form: username@proxy.uni.edu:port. Click 'Ok'. Repeat the process to create the variable HTTPS_PROXY. Click 'Ok'.
 Check for Updates should now work.
Why is SasView no longer able to run my existing custom models?
 There may be two reasons for this. From release 4.0 the structure of plugin models altered considerably in order to provide the enhanced functionality (access to polydispersity, etc) many of our users had demanded. Though we have attempted to provide some degree of internal translation, the reality is that this will likely only be truly successful for simple models. We recommend that old models be rewritten to comply with the requirements of the new architcture. New models can then be contributed through our new Model Marketplace.
 The last vestiges of the old SansView name leftover from the genesis of the application were removed for release 3.1. While most visible parts of the application had already been converted to sasview prior to version 3.0, some of the packages still had the prefix "sans", including unfortunately the package used in custom models "sans.models.pluginmodel." In order to fix this simply apply the following fix. We appologize for the inconvenience.
 under Fitting > Edit Custom Model > Advanced (on the menu bar) open the offending custom model. Find any import statements (should be at the beginning) including the from sans.models.pluginmodel import Model1DPlungin and replace "sans" with "sas." Save the model. It should now work
How can I see the separate contributions of the form factor and structure factor?
SasView 2.x.x / SasView 3.x.x / SasView 4.0.x:
 Load the data and perform the fit as normal, including the structure factor. Go to the menubar and select Edit > Copy Params....
 Then open two new Fit Pages (Fitting > New Fit Page). In one of the new Fit Pages select the form factor model you used in the first Fit Page, in the other new Fit Page select the structure factor model you used in the first Fit Page.
 In each of the two new Fit Pages, select Edit > Paste Params....
 NB: depending on the version of SasView you are using (4.0.x versus earlier versions), it may be necessary to manually adjust the scale factors or volume fraction parameter so that only one or other reports the volume fraction.
 In each of the two new Fit Pages, click Compute. The form factor and structure factor will appear as Theories in the Data Explorer panel.
SasView 4.1 and later:
 Once a fit has been performed P(Q), and S(Q) if included in the fit, will appear under Theories in the Data Explorer panel.
How can I include a structure factor in a model?
 There are two ways to include an S(Q) in a model:
 By clicking on the S(Q) dropdown box adjacent to the P(Q) model choice in a FitPage
 Using the SumMulti function under Custom/Plugin Model operations.
 However, these two methods are not equivalent.
 When you choose to implement S(Q) from the FitPage, the effective radius for the particle described by P(Q) is calculated automatically and the volume fraction used in S(Q) is set to be the volume fraction of the particles (ie, the 'scale' parameter). Behind the scenes, the model calculates the radius of a sphere that would have the same second virial coefficient as the chosen particles and uses that in the S(Q) calculation as the effective radius for the interaction potential. This method works well for particles with an axial ratio up to about a 3to1 but beyond that is not correct.
 When you make a Custom/Plugin model, the full set of parameters for calculating S(Q) are made available and you need to have an idea of what the effective radius and effective volume fraction for your chosen particles should be.
 All S(Q) functions available in SasView were derived for spherical particles.
 Also be aware that if you create a Custom/Plugin model from two other Custom/Plugin models with S(Q) included, for example, something like P1(Q)S1(Q) + P2(Q)S2(Q), SasView will only take account of the 'self' interactions. If you also need to properly account for the 'cross' interactions you will need to program a Custom/Plugin model from first principles.
What is the Model Marketplace?
 The Model Marketplace is an open repository of SasView fitting models we have set up to allow the SAS Community to contribute and share models.
 Despite its name, it is free to use!
Question?