Opened 7 months ago

Last modified 6 months ago

## #895 new defect

# user defined fit parameters for constrained fitting

Reported by: | pkienzle | Owned by: | |
---|---|---|---|

Priority: | major | Milestone: | SasView Next Release +1 |

Component: | SasView | Keywords: | |

Cc: | Work Package: | SasView GUI Enhancements |

### Description

Sometimes users need to be able to create additional fitted parameters used to define other parameters in the system. For example, to fit the volume fraction of an sld mixture where the volume fraction is the same across the entire series, but the slds of the mixture components are different across the series, you want to define a parameter Vf and set:

M1.sld = Vf*A1 + (1-Vf)*B1 M2.sld = Vf*A2 + (1-Vf)*B2 ...

where A1, B1, A2, B2, ... are known values. You then want to set limits on Vf to (0,1) and set it as a fitting parameter.

### Change History (2)

### comment:1 Changed 7 months ago by pkienzle

### comment:2 Changed 6 months ago by richardh

Also this could for example specify the number of points or convergence criterion to be used in an integration scheme for P(Q), if possible to return the last computed value of a derived parameter such as Debye length, or provide the value of a fixed constant used in the model calculation.

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You can do this now, with some trickery:

Because the returned value is independent of the parameters, the new fit page will not change the shape of the posterior probability distribution. The reported chisq will be slightly less than expected because the number of degrees of freedom is increased by the 2 fake data points, however, this will not be noticeable since DOF is already very large because of the number of points in SAS datasets.

Note that new parameter cannot be polydisperse.