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Last modified: December 2013

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AHELP for CIAO 4.9 Sherpa v1


Context: plotting


Access the simulation results of the likelihood ratio test.




The get_pvalue_results() command returns the likelihood ratio test results computed by the plot_pvalue() command, comparing fits of the null model to fits of the alternative model using faked data with Poisson noise. The likelihood ratio based on the observed data is returned, along with the p-value, used to reject or accept the null model.

The following attributes are returned by get_pvalue_results():

  • samples - parameter samples from all iterations, in the order of the parameter list (as many as samples as specified in the 'num' parameter of plot_pvalue() are returned; default is 500)
  • stats - calculated fit statistics in all iterations (first column null, alt second)
  • ratios - calculated likelihood ratios from all iterations
  • null - fit statistic of null on real data
  • alt - fit statistic of alt on real data
  • lr - likelihood ratio on real data
  • ppp - p-value

Example 1

sherpa> plot_pvalue(powlaw1d.p1, p1+gauss1d.g1)
sherpa> print get_pvalue_results()
samples = [[  8.5623e-01   5.6209e+03]
 [  8.5391e-01   5.5819e+03]
 [  8.5695e-01   5.7627e+03]
 [  8.6336e-01   5.9058e+03]
 [  8.5459e-01   5.6061e+03]
 [  8.3418e-01   4.9820e+03]]
stats   = [[ 466.1517  466.1517]
 [ 500.005   500.005 ]
 [ 474.0595  474.0595]
 [ 424.5051  424.5051]
 [ 453.3375  453.3375]
 [ 463.3019  463.3019]]
ratios  = [-0. -0. -0. ..., -0. -0. -0.]
null    = 3958.4582610638136
alt     = 3958.4582610638136
lr      = -0.0
ppp     = 0.054

Call the get_pvalue_results() command with no arguments to print to the screen the results of the likelihood ratio test performed by the plot_pvalue() command. View the array of ratios comparing fits to simulated data done with a simple (null) power-law model versus the more-complex (alternative) power-law plus Gaussian model, along with the likelihood ratio and p-value based on the observed data.

Example 2

lrt = get_pvalue_results()

Store the data returned by get_pvalue_results() to variable "lrt", in order to access and manipulate individual attributes within a script.

sherpa> res.ratios
            array([-0., -0., -0., ..., -0., -0., -0.])

sherpa> res.ppp

sherpa> res.stats
array([[ 466.1517,  466.1517],
       [ 500.005 ,  500.005 ],
       [ 474.0595,  474.0595],
       [ 424.5051,  424.5051],
       [ 453.3375,  453.3375],
       [ 463.3019,  463.3019]])

sherpa> len(res.stats)

sherpa> res.stats[46]
            array([ 532.5428,  532.5428])


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

See Also

get_data_prof, get_data_prof_prefs, get_delchi_prof, get_delchi_prof_prefs, get_fit_prof, get_model_prof, get_model_prof_prefs, get_resid_prof, get_resid_prof_prefs, get_source_prof, get_source_prof_prefs, plot_chart_spectrum, prof_data, prof_delchi, prof_fit, prof_fit_delchi, prof_fit_resid, prof_model, prof_resid, prof_source
get_arf_plot, get_bkg_plot
list_model_ids, show_bkg_model, show_bkg_source
add_model, add_user_pars, clean, create_model_component, delete_bkg_model, delete_model, delete_model_component, get_model, get_model_autoassign_func, get_model_component, get_model_component_image, get_model_component_plot, get_model_plot, get_num_par, get_order_plot, get_par, get_pileup_model, get_source, get_source_component_image, get_source_component_plot, image_model, image_model_component, image_source, image_source_component, integrate, integrate1d, link, load_table_model, load_template_model, load_user_model, normal_sample, reset, save_model, save_source, set_bkg_model, set_full_model, set_model_autoassign_func, set_pileup_model, set_source, set_xsabund, set_xscosmo, set_xsxsect, set_xsxset, t_sample, uniform_sample
get_energy_flux_hist, get_lrt_plot, get_lrt_results, get_photon_flux_hist, get_pvalue_plot, get_split_plot, plot, plot_arf, plot_bkg, plot_cdf, plot_chisqr, plot_data, plot_delchi, plot_energy_flux, plot_fit, plot_model, plot_model_component, plot_order, plot_pdf, plot_photon_flux, plot_pvalue, plot_ratio, plot_resid, plot_scatter, plot_source, plot_source_component, plot_trace, set_xlinear, set_xlog, set_ylinear, set_ylog
delete_psf, load_conv, plot_kernel
save_delchi, save_resid
get_chisqr_plot, get_delchi_plot
calc_chisqr, calc_energy_flux, calc_model_sum, calc_photon_flux, calc_source_sum, calc_stat, eqwidth
contour_model, contour_ratio, contour_resid, get_ratio, get_resid

Last modified: December 2013
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