3,409 research outputs found
Probabilistic Gradients for Fast Calibration of Differential Equation Models
Calibration of large-scale differential equation models to observational or
experimental data is a widespread challenge throughout applied sciences and
engineering. A crucial bottleneck in state-of-the art calibration methods is
the calculation of local sensitivities, i.e. derivatives of the loss function
with respect to the estimated parameters, which often necessitates several
numerical solves of the underlying system of partial or ordinary differential
equations. In this paper we present a new probabilistic approach to computing
local sensitivities. The proposed method has several advantages over classical
methods. Firstly, it operates within a constrained computational budget and
provides a probabilistic quantification of uncertainty incurred in the
sensitivities from this constraint. Secondly, information from previous
sensitivity estimates can be recycled in subsequent computations, reducing the
overall computational effort for iterative gradient-based calibration methods.
The methodology presented is applied to two challenging test problems and
compared against classical methods
A Fourier representation of kernel Stein discrepancy with application to Goodness-of-Fit tests for measures on infinite dimensional Hilbert spaces
Kernel Stein discrepancy (KSD) is a widely used kernel-based measure of
discrepancy between probability measures. It is often employed in the scenario
where a user has a collection of samples from a candidate probability measure
and wishes to compare them against a specified target probability measure. KSD
has been employed in a range of settings including goodness-of-fit testing,
parametric inference, MCMC output assessment and generative modelling. However,
so far the method has been restricted to finite-dimensional data. We provide
the first analysis of KSD in the generality of data lying in a separable
Hilbert space, for example functional data. The main result is a novel Fourier
representation of KSD obtained by combining the theory of measure equations
with kernel methods. This allows us to prove that KSD can separate measures and
thus is valid to use in practice. Additionally, our results improve the
interpretability of KSD by decoupling the effect of the kernel and Stein
operator. We demonstrate the efficacy of the proposed methodology by performing
goodness-of-fit tests for various Gaussian and non-Gaussian functional models
in a number of synthetic data experiments.Comment: To appear in Bernoull
Statistical Inference for Generative Models with Maximum Mean Discrepancy
While likelihood-based inference and its variants provide a statistically
efficient and widely applicable approach to parametric inference, their
application to models involving intractable likelihoods poses challenges. In
this work, we study a class of minimum distance estimators for intractable
generative models, that is, statistical models for which the likelihood is
intractable, but simulation is cheap. The distance considered, maximum mean
discrepancy (MMD), is defined through the embedding of probability measures
into a reproducing kernel Hilbert space. We study the theoretical properties of
these estimators, showing that they are consistent, asymptotically normal and
robust to model misspecification. A main advantage of these estimators is the
flexibility offered by the choice of kernel, which can be used to trade-off
statistical efficiency and robustness. On the algorithmic side, we study the
geometry induced by MMD on the parameter space and use this to introduce a
novel natural gradient descent-like algorithm for efficient implementation of
these estimators. We illustrate the relevance of our theoretical results on
several classes of models including a discrete-time latent Markov process and
two multivariate stochastic differential equation models
A High-dimensional Convergence Theorem for U-statistics with Applications to Kernel-based Testing
We prove a convergence theorem for U-statistics of degree two, where the data dimension d
is allowed to scale with sample size n
. We find that the limiting distribution of a U-statistic undergoes a phase transition from the non-degenerate Gaussian limit to the degenerate limit, regardless of its degeneracy and depending only on a moment ratio. A surprising consequence is that a non-degenerate U-statistic in high dimensions can have a non-Gaussian limit with a larger variance and asymmetric distribution. Our bounds are valid for any finite n
and d
, independent of individual eigenvalues of the underlying function, and dimension-independent under a mild assumption. As an application, we apply our theory to two popular kernel-based distribution tests, MMD and KSD, whose high-dimensional performance has been challenging to study. In a simple empirical setting, our results correctly predict how the test power at a fixed threshold scales with d
and the bandwidth
System Effects in Identifying Risk-Optimal Data Requirements for Digital Twins of Structures
Structural Health Monitoring (SHM) technologies offer much promise to the
risk management of the built environment, and they are therefore an active area
of research. However, information regarding material properties, such as
toughness and strength is instead measured in destructive lab tests. Similarly,
the presence of geometrical anomalies is more commonly detected and sized by
inspection. Therefore, a risk-optimal combination should be sought,
acknowledging that different scenarios will be associated with different data
requirements. Value of Information (VoI) analysis is an established statistical
framework for quantifying the expected benefit of a prospective data collection
activity. In this paper the expected value of various combinations of
inspection, SHM and testing are quantified, in the context of supporting risk
management of a location of stress concentration in a railway bridge. The Julia
code for this analysis (probabilistic models and influence diagrams) is made
available. The system-level results differ from a simple linear sum of marginal
VoI estimates, i.e. the expected value of collecting data from SHM and
inspection together is not equal to the expected value of SHM data plus the
expected value of inspection data. In summary, system-level decision making,
requires system-level models
Incorporating habitat distribution in wildlife disease models: conservation implications for the threat of squirrelpox on the Isle of Arran
Emerging infectious diseases are a substantial threat to native populations. The spread of disease through naive native populations will depend on both demographic and disease parameters, as well as on habitat suitability and connectivity. Using the potential spread of squirrelpox virus (SQPV) on the Isle of Arran as a case study, we develop mathematical models to examine the impact of an emerging disease on a population in a complex landscape of different habitat types. Furthermore, by considering a range of disease parameters, we infer more generally how complex landscapes interact with disease characteristics to determine the spread and persistence of disease. Specific findings indicate that a SQPV outbreak on Arran is likely to be short lived and localized to the point of introduction allowing recovery of red squirrels to pre-infection densities; this has important consequences for the conservation of red squirrels. More generally, we find that the extent of disease spread is dependent on the rare passage of infection through poor quality corridors connecting good quality habitats. Acute, highly transmissible infectious diseases are predicted to spread rapidly causing high mortality. Nonetheless, the disease typically fades out following local epidemics and is not supported in the long term. A chronic infectious disease is predicted to spread more slowly but can remain endemic in the population. This allows the disease to spread more extensively in the long term as it increases the chance of spread between poorly connected populations. Our results highlight how a detailed understanding of landscape connectivity is crucial when considering conservation strategies to protect native species from disease threats
Nanosecond laser texturing for high friction applications
AbstractA nanosecond pulsed Nd:YAG fibre laser with wavelength of 1064nm was used to texture several different steels, including grade 304 stainless steel, grade 316 stainless steel, Cr–Mo–Al ‘nitriding’ steel and low alloy carbon steel, in order to generate surfaces with a high static friction coefficient. Such surfaces have applications, for example, in large engines to reduce the tightening forces required for a joint or to secure precision fittings easily. For the generation of high friction textures, a hexagonal arrangement of laser pulses was used with various pulse overlaps and pulse energies. Friction testing of the samples suggests that the pulse energy should be high (around 0.8mJ) and the laser pulse overlap should be higher than 50% in order to achieve a static friction coefficient of more than 0.5. It was also noted that laser processing increases the surface hardness of samples which appears to correlate with the increase in friction. Energy-Dispersive X-ray spectroscopy (EDX) measurements indicate that this hardness is caused by the formation of hard metal-oxides at the material surface
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