AHELP for CIAO 4.16 Sherpa

# set_prior

Context: statistics

## Synopsis

Set the prior function to use with a parameter.

## Syntax

```set_prior(par, prior)

par - a `Parameter` instance
prior - function or sherpa.models.model.Model instance```

## Description

The default prior used by `get_draws` for each parameter is flat, varying between the soft minimum and maximum values of the parameter (as given by the `min` and `max` attributes of the parameter object). The `set_prior` function is used to change the form of the prior for a parameter, and `get_prior` returns the current prior for a parameter.

## Examples

### Example 1

Set the prior for the kt parameter of the therm component to be a gaussian, centered on 1.7 keV and with a FWHM of 0.35 keV:

```>>> create_model_component('xsapec', 'therm')
>>> create_model_component('gauss1d', 'p_temp')
>>> p_temp.pos = 1.7
>>> p_temp.fwhm = 0.35
>>> set_prior(therm.kT, p_temp)```

### Example 2

Create a function ( lognorm ) and use it as the prior of the nh parameter of the abs1 model component:

```>>> def lognorm(x):
...     nh = 20
...     sigma = 0.5  # use a sigma of 0.5
...     # nH is in units of 10^-22 so convert
...     dx = np.log10(x) + 22 - nh
...     norm = sigma / np.sqrt(2 * np.pi)
...     return norm * np.exp(-0.5 * dx * dx / (sigma * sigma))
...
>>> create_model_component('xsphabs', 'abs1')
>>> set_prior(abs1.nH, lognorm)```

### PARAMETERS

The parameters for this function are:

Parameter Definition
par A parameter of a model instance.
prior The function to use for a prior. It must accept a single argument and return a value of the same size as the input.

## Bugs

See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.

confidence
set_conf_opt, set_covar_opt, set_proj_opt
data
set_areascal, set_arf, set_backscal, set_bkg, set_coord, set_counts, set_data, set_dep, set_exposure, set_grouping, set_quality, set_rmf, set_staterror, set_syserror
filtering
set_filter
info
list_stats
methods
get_draws, set_iter_method, set_iter_method_opt, set_method, set_method_opt
modeling
get_par, get_xsabund, get_xscosmo, get_xsxsect, get_xsxset, set_bkg_model, set_bkg_source, set_full_model, set_model, set_par, set_pileup_model, set_source, set_xsabund, set_xscosmo, set_xsxsect, set_xsxset
statistics
cash, chi2constvar, chi2datavar, chi2gehrels, chi2modvar, chi2xspecvar, chisquare, cstat, get_prior, leastsq, list_priors, set_sampler, set_sampler_opt, set_stat, wstat
utilities
set_analysis, set_default_id
visualization
image_setregion