Importance sampling (reweighting) an analysis

Often we want to perform the analysis with a simplified model, e.g., using an ROQ approximation or a less-sophisticated waveform approximant. However, we also want to know what result we would have got without the simplifying assumptions.

In order to facilitate this, we provide an option to specify modifications to the run configuration as an additional argument ----reweighting-configuration. This should be either a json file or string containing a new prior-file or any of the likelihood arguments that can be passed to the bilby_pipe parser.

If you are specifying a new prior file, the parameterization should remain the same, reweighting to include new parameters will generally not work with this implementation and should be done on a case-by-case basis. The exception to this is adding calibration marginalization which can be included by specifying a new calibration-model as described in arXiv:2009.10193.

If you are using the file transfer option, you must list this configuration file and any other needed files, e.g., a new prior file/calibration envelopes.

Reweighting nested samples

If the initial sampling is done with a nested sampler, it is possible to apply the importance sampling directly to the nested samples. This can lead to larger reweighting efficiency as the nested samples probe the tails of the posterior more deeply. To enable this, set reweight-nested-samples=True.

Example using the configuration file

In this example, we perform the initial analysis with the “relative binning” method and a waveform without higher-order emission modes (IMRPhenomXAS) and then we reweight to the full likelihood with a waveform with higher-order modes (IMRPhenomXHM).

The configuration file for the initial analysis is a modified version of the injection example in the examples page.

spline-calibration-amplitude-uncertainty-dict {H1: 0.1, L1: 0.1}
spline-calibration-phase-uncertainty-dict {H1: 0.1, L1: 0.1}
calibration-lookup-table = {H1: outdir_bbh_injection/data/calibration_H1.h5, L1: outdir_bbh_injection/data/calibration_L1.h5}

accounting = ligo.dev.o4.cbc.pe.bilby

label = bbh_injection
outdir = outdir_bbh_injection

detectors = [H1, L1]
duration = 64

sampler = dynesty
sampler-kwargs = {'nlive': 1000}
additional-transfer-paths = outdir_bbh_injection/data/calibration_H1.h5,outdir_bbh_injection/data/calibration_L1.h5

prior-file = bbh_injection.prior

injection = True
gaussian-noise = True
injection-dict = {"mass_1": 41, 'mass_2': 39, 'a_1': 0, 'a_2': 0, 'tilt_1': 0, 'tilt_2': 0,
    'phi_12': 0, 'phi_jl': 0, 'luminosity_distance': 1000, 'dec': -0.2, 'ra': 1.4,
    'theta_jn': 0.2, 'psi': 0, 'phase': 0, 'geocent_time': 0}
injection-waveform-approximant=IMRPhenomXHM
n-simulation = 1
n-parallel = 1
request-cpus = 4
osg = True

likelihood-type = bilby.gw.likelihood.relative.RelativeBinningGravitationalWaveTransient
fiducial_parameters = {"mass_1": 41, 'mass_2': 39, 'a_1': 0, 'a_2': 0, 'tilt_1': 0, 'tilt_2': 0,
    'phi_12': 0, 'phi_jl': 0, 'luminosity_distance': 1000, 'dec': -0.2, 'ra': 1.4,
    'theta_jn': 0.2, 'psi': 0, 'phase': 0, 'geocent_time': 0}
waveform-approximant = "IMRPhenomXAS"
frequency-domain-source-model = bilby.gw.source.lal_binary_black_hole

reweighting-configuration=reweight.json
reweight-nested-samples=False

Since we are changing the likelihood used, we need to specify a new likelihood-type and frequency-domain-waveform-model. To change the waveform approximant, we just specify the new model.

{
    "likelihood-type": "bilby.gw.likelihood.GravitationalWaveTransient",
    "waveform_approximant": "IMRPhenomXHM",
    "calibration-model": "CubicSpline",
    "calibration-marginalization": true,
    "number-of-response-curves": 1000
}

Example for a completed analysis

It is also possible to reweight a result for an analysis that has fully completed without having to modify the configuration. In this case you can use the bilby_pipe_reweight_result executable.