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# Summary
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In a typical Bayesian inference problem, the data likelihood is not known. However, in recent years, machine learning methods for density estimation can allow for inference using an estimator of the data likelihood. This likelihood is created with neural networks that are trained on simulations - one of the many tools for simulation based inference (SBI, @sbi). In such analyses, density-estimation simulation-based inference methods can derive a posterior, which typically involves
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In a typical Bayesian inference problem, the data likelihood is not known. However, in recent years, machine learning methods for density estimation can allow for inference using an estimator of the data likelihood. This likelihood is created with neural networks that are trained on simulations - one of the many tools for Simulation Based Inference (SBI), [@sbi]. In such analyses, density-estimation simulation-based inference methods can derive a posterior, which typically involves
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* simulating a set of data and model parameters $\{(\boldsymbol{\xi}, \boldsymbol{\pi})_0, ..., (\boldsymbol{\xi}, \boldsymbol{\pi})_N\}$,
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* simulating a set of data and model parameters $\{(\boldsymbol{\xi}, \boldsymbol{\pi})_0, \ldots, (\boldsymbol{\xi}, \boldsymbol{\pi})_N\}$,
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* obtaining a measurement $\hat{\boldsymbol{\xi}}$,
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* compressing the simulations and the measurements - usually with a neural network or linear compression - to a set of summaries $\{(\boldsymbol{x}, \boldsymbol{\pi})_0, ..., (\boldsymbol{x}, \boldsymbol{\pi})_N\}$ and $\hat{\boldsymbol{x}}$,
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* compressing the simulations and the measurements - usually with a neural network or linear compression - to a set of summaries $\{(\boldsymbol{x}, \boldsymbol{\pi})_0, \ldots, (\boldsymbol{x}, \boldsymbol{\pi})_N\}$ and $\hat{\boldsymbol{x}}$,
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* fitting an ensemble of normalising flow or similar density estimation algorithms (e.g. a Gaussian mixture model),
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* the optional optimisation of the parameters for the architecture and fitting hyperparameters of the algorithms,
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* the optional optimisation of the parameters for the architecture and fitting hyper-parameters of the algorithms,
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* sampling the ensemble posterior (using an MCMC sampler if the likelihood is fit directly), conditioned on the data-vector, to obtain parameter constraints on the parameters of a physical model, $\boldsymbol{\pi}$.
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`sbiax` is a code for implementing each of these steps. The code allows for Neural Likelihood Estimation [@papamakarios; @delfi] and Neural Posterior Estimation [@npe].
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`sbiax` is a software package that implements each of these steps. The code allows for Neural Likelihood Estimation [@papamakarios; @delfi] and Neural Posterior Estimation [@npe].
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As shown in [@homersbi], SBI can successfully obtain the correct posterior widths and coverages given enough simulations which agree with the analytic solution - this software was used in the research for this publication.
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As shown in @homersbi, SBI can successfully obtain the correct posterior widths and coverages given enough simulations which agree with the analytic solution - this software was used in the research for this publication.
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# Statement of need
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Simulation-based inference (SBI) covers a broad class of statistical techniques such as Approximate Bayesian Computation (ABC,[@ABC]), Neural Ratio Estimation (NRE, [@NRE]), Neural Likelihood Estimation (NLE) and Neural Posterior Estimation (NPE). These techniques can derive posterior distributions conditioned of noisy data vectors in a rigorous and efficient manner with assumptions on the data likelihood. In particular, density-estimation methods have emerged as a promising method, given their efficiency, using generative models to fit likelihoods or posteriors directly using simulations.
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Simulation Based Inference (SBI) covers a broad class of statistical techniques such as Approximate Bayesian Computation (ABC)[@ABC], Neural Ratio Estimation (NRE), [@NRE], Neural Likelihood Estimation (NLE) and Neural Posterior Estimation (NPE). These techniques can derive posterior distributions conditioned of noisy data vectors in a rigorous and efficient manner with assumptions on the data likelihood. In particular, density-estimation methods have emerged as a promising method, given their efficiency, using generative models to fit likelihoods or posteriors directly using simulations.
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In the field of cosmology, SBI is of particular interest due to complexity and non-linearity of models for the expectations of non-standard summary statistics of the large-scale structure, as well as the non-Gaussian noise distributions for these statistics. The assumptions required for the complex analytic modelling of these statistics as well as the increasing dimensionality of data returned by spectroscopic and photometric galaxy surveys limits the amount of information that can be obtained on fundamental physical parameters. Therefore, the study and research into current and future statistical methods for Bayesian inference is of paramount importance for the cosmology, especially in light of current and next-generation survey missions such as DES [@Euclid], DESI [@DESI] and Euclid [@Euclid].
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In the field of cosmology, SBI is of particular interest due to complexity and non-linearity of models for the expectations of non-standard summary statistics of the large-scale structure, as well as the non-Gaussian noise distributions for these statistics. The assumptions required for the complex analytic modelling of these statistics as well as the increasing dimensionality of data returned by spectroscopic and photometric galaxy surveys, limit the amount of information that can be obtained on fundamental physical parameters. Therefore, the study and research into current and future statistical methods for Bayesian inference is of paramount importance for the cosmology, especially in light of current and next-generation survey missions such as DES [@Euclid], DESI [@DESI], and Euclid [@Euclid].
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The software we present, `sbiax`, is designed to be used by machine learning and physics researchers for running Bayesian inferences using density-estimation SBI techniques. These models can be fit easily with multi-accelerator training and inference within the code. This software - written in `jax`[@jax] - allows for seemless integration of cutting edge generative models to SBI, including continuous normalising flows [@ffjord], matched flows [@flowmatching], masked autoregressive flows [@mafs; @flowjax] and Gaussian mixture models - all of which are implemented in the code. The code features integration with the `optuna`[@optuna]hyperparameter optimisation framework which would be used to ensure consistent analyses, `blackjax`[@blackjax] for fast MCMC sampling and `equinox`[@equinox] for neural network methods. The design of `sbiax` allows for new density estimation algorithms to be trained and sampled from, as long as they conform to a simple and typical design pattern demonstrated in `sbiax`.
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The software we present, `sbiax`, is designed to be used by machine learning and physics researchers for running Bayesian inferences using density-estimation SBI techniques. These models can be fit easily with multi-accelerator training and inference within the code. This software - written in `jax`[@jax] - allows for seemless integration of cutting edge generative models to SBI, including continuous normalising flows [@ffjord], matched flows [@flowmatching], masked autoregressive flows [@mafs; @flowjax], and Gaussian mixture models - all of which are implemented in the code. The code features integration with the `optuna`[@optuna]hyper-parameter optimisation framework which would be used to ensure consistent analyses, `blackjax`[@blackjax] for fast MCMC sampling, and `equinox`[@equinox] for neural network methods. The design of `sbiax` allows for new density estimation algorithms to be trained and sampled from, as long as they conform to a simple and typical design pattern demonstrated in `sbiax`.
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Whilst excellent software packages already exist for conducting simulation-based inference (e.g. `sbi`[@sbimacke], `sbijax`[@sbidirmeier]) for some applications it is useful to have a lightweight implementation that focuses on speed, ensembling of density estimators and easily integrated MCMC sampling (e.g. for ensembles of likelihoods) - all of which is based on a lightweight and regularly maintained `jax` machine learning library such as `equinox`[@equinox]. `sbiax` depends on density estimators and compression modules - as long as log-probability and callable methods exists for these, they can be integrated seemlessly.
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@@ -82,7 +82,7 @@ This density estimate is fit to a set of $N$ simulation-parameter pairs $\{(\bol
where $q(\boldsymbol{x}|\boldsymbol{\pi})$ is the unknown likelihood from which the simulations $\boldsymbol{x}$ are drawn. This applies similarly for an estimator of the posterior (instead of the likelihood as shown here) and is the basis of being able to estimate the likelihood or posterior directly when an analytic form is not available. If the likelihood is fit from simulations, a prior is required and the posterior is sampled via an MCMC-sampelr given some measurement. This is implemented within the code.
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where $q(\boldsymbol{x}|\boldsymbol{\pi})$ is the unknown likelihood from which the simulations $\boldsymbol{x}$ are drawn. This applies similarly for an estimator of the posterior (instead of the likelihood as shown here) and is the basis of being able to estimate the likelihood or posterior directly when an analytic form is not available. If the likelihood is fit from simulations, a prior is required and the posterior is sampled via an MCMC-sampler given some measurement. This is implemented within the code.
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An ensemble of density estimators (with parameters - e.g. the weights and biases of the networks - denoted by $\{ \phi_0, ..., \phi_J\}$) has a likelihood which is written as
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