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ominusliticus committed Jan 30, 2024
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Expand Up @@ -38,23 +38,27 @@ bibliography: references.bib

Uncertainty quantification using Bayesian methods is a growing area of
research. Bayesian model mixing (BMM) is a recent development which
combines the predictions from multiple models such that each model's
best qualities are preserved in the final result. Practical tools and
combines the predictions from multiple models such that each the fidelity
of individual models is preserved in the final result. Practical tools and
analysis suites that facilitate such methods are therefore needed.
`Taweret` introduces BMM to existing Bayesian uncertainty quantification
`Taweret`[^1] introduces BMM to existing Bayesian uncertainty quantification
efforts. Currently `Taweret` contains three individual Bayesian model
mixing techniques, each pertaining to a different type of problem
structure; we encourage the future inclusion of user-developed mixing
methods. `Taweret`'s first use case is in nuclear physics, but the
package has been structured such that it should be adaptable to any
research engaged in model comparison or model mixing.

[^1]: Taweret is the Egyptian goddess, known as the protector of children and women,
whose body is a fusion of a hippo, lion and crocodile which represent her ferocity.
Similarly, `Taweret`, the package, seeks to fuse models together to represent
observed phenomena.

# Statement of need

In physics applications, multiple models with different physics
assumptions can be used to describe an underlying system of interest. It
is usually the case that each model has varying fidelity to the observed
process across the input domain. Though each model may have similar
assumptions can be used to describe an underlying system of interest.
Though each model may have similar
predictive accuracy on average, the fidelity of the approximation across
a subset of the domain may differ drastically for each of the models
under consideration. In such cases, inference and prediction based on a
Expand All @@ -79,6 +83,10 @@ where $p(Y_0 \mid x_0, Y, \mathcal{M}_k)$ represents the predictive density of a
setup, a key challenge is defining $w_k(x)$---the functional
relationship between the inputs and the weights.

![Schematic of Bayesian model mixing. Each model has region of paramtere space where it has a high
fidelity, but all models are meant to describe the same phenomenon. To obtain a model that works
well for all of parameter space, we combine them using Bayesian model mixing methods](bmm_schematic.pdf){#fig:bmm_schematic width="\\textwidth"}

This work introduces `Taweret`, a Python package for Bayesian model
mixing that includes three novel approaches for combining models, each
of which defines the weight function in a unique way (see
Expand Down Expand Up @@ -146,8 +154,20 @@ applications in relativistic heavy-ion collision physics can be found in
the Ph.D. thesis of D. Liyanage [@Liyanage_thesis]. The bivariate linear
mixing method can mix two models either using a density-mixing or a
mean-mixing strategy. Currently, this is the only mixing method in
`Taweret` that can also calibrate the models while mixing. It allows the
user to choose among the following mixing functions:
`Taweret` that can also calibrate the models while mixing.
Each physics-based model we consider may have unknown parameters which have
physical meaning.
In this context, Bayesian calibration corresponds to the process of using
observational dat to learn the values (and more generally, the posterior
distribtutions) of this these unknown parameters.
Most approaches in model mixing and model averaging use a two-step approach:
(step 1) fit individual models using a subset of the data; (step 2) mix the
predictions from each model (the results from step 1) using the other subset
of the data to learn the weights.
Therefore, this joint calibration of and mixing idea looks to do everything at
once, rather than use the two step process.

The user may choose among the following mixing functions:

- step: $\Theta(\beta_0-x)$

Expand Down Expand Up @@ -330,6 +350,14 @@ this growing framework, we hope to enable continuous integration
routines for individuals contributing and create docker images that will
run `Taweret`.

# Contributions
All authors have contributed to the development of the Taweret framework,
while individual mixing methods were developed by
D. Liyanage (bivariate linear mixing), A. Semposki (multivariate model
mixing), and J. Yanotty (additive regression trees).
Ongoing work by K. Ingles will be included in the next version release
of `Taweret`.

# Disclosure statement

The authors of this work are not aware of any conflicts of interest that
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