Accelerating Asymptotically Exact MCMC for Computationally Intensive Models via Local Approximations

We construct a new framework for accelerating Markov chain Monte Carlo in posterior sampling problems where standard methods are limited by the computational cost of the likelihood, or of numerical models embedded therein. Our approach introduces local approximations of these models into the Metropolis-Hastings kernel, borrowing ideas from deterministic approximation theory, optimization, and experimental design. Previous efforts at integrating approximate models into inference typically sacrifice either the sampler's exactness or efficiency; our work seeks to address these limitations by exploiting useful convergence characteristics of local approximations. We prove the ergodicity of our approximate Markov chain, showing that it samples asymptotically from the \emph{exact} posterior distribution of interest. We describe variations of the algorithm that employ either local polynomial approximations or local Gaussian process regressors. Our theoretical results reinforce the key observation underlying this paper: when the likelihood has some \emph{local} regularity, the number of model evaluations per MCMC step can be greatly reduced without biasing the Monte Carlo average. Numerical experiments demonstrate multiple order-of-magnitude reductions in the number of forward model evaluations used in representative ODE and PDE inference problems, with both synthetic and real data.Comment: A major update of the theory and example

“Supply and demand trends for fertilizer in Zimbabwe: 1930 to date”: Key drivers and lessons learnt

The fertilizer sector in Zimbabwe has evolved over the years in response to different policy changes based on the government’s priorities on agricultural development. The industry grew from the 1930s that targeted primarily large scale commercial farmers, through the liberalization period of the mid 1990s, and recent changes that have introduced controls on the marketing system. Since 2000, following the fast track land reform program, Zimbabwe has faced food insecurity challenges that have been exacerbated by the political and economic crises. This prompted the government to adopt policies that have reduced private sector interests in fertilizer supply. In this situation where explicit fertilizer markets have been absent, relief programs have been leading in facilitating deliveries of fertilizers to poorer smallholder farmers located even in remote areas. The supply of fertilizers in Zimbabwe has been driven by government policy, finance and infrastructure while the demand has primarily been a function of farmer’s capacity to acquire fertilizers, availability of water and farmers knowledge of fertilizer use. There is need for a policy shift that promotes a competitive fertilizer marketing to support a broader range of farmers in Zimbabwe leading to agricultural productivity growth. Investment in infrastructure is critical to reduce marketing costs and to boost fertilizer demand; policies that strengthen farmer’s capacity to acquire fertilizers and increase their knowledge on fertilizer use complemented by technologies that promote water use efficiencies are needed.Fertilizer, supply and demand, policy reform, consumption trends, Crop Production/Industries,

Two new skinks (Scincidae: Glaphyromorphus) from rainforest habitats in north-eastern Australia

Tropical rainforest is largely restricted in Australia to the fairly continuous Wet Tropics region and disconnected patches to the north on Cape York. The Wet Tropics is relatively well explored and studied, whereas the rainforests of Cape York have received less attention due to their remoteness. Here we describe two new species of Glaphyromorphus skinks from rainforest areas on Cape York. The two new species are most similar to each other and to G. fuscicaudis and G. nigricaudis, but both are readily diagnosed on numerous traits. Glaphyromorphus othelarrni sp. nov. is diagnosed from all similar species by its supralabial count (typically 8 vs 7), high number of subdigital lamellae beneath the 4th finger (14-15 vs 85 mm) and slender body shape, low number of subdigital lamellae beneath the 4th toe (17-20 vs generally 20 or more), and head and body pattern. Both species also differ from each other and similar congeners in other aspects of body shape, scalation and colour pattern. Glaphyromorphus othelarrni sp. nov. is restricted to boulder-strewn rainforest of the Melville Range, whilst Glaphyromorphus nyanchupinta sp. nov. is known only from upland rainforest in the McIlwraith Range. We discuss patterns of rainforest vertebrate endemism on Cape York, and the importance of lithorefugia in generating these

A new genus to accommodate three skinks currently assigned to Menetia (Lacertilia: Scincidae)

Menetia timlowi Ingram 1977 has had a chequered generic history due to ambiguity in interpreting its various character states. Ingram (1977) assigned this species to Menetia on the strength of “the long narrow obliquely oriented first supraocular” and “enlarged upper circumoculars”. In doing so, he admitted the possibility that M. timlowi “may in fact be a Carlia” but because “all Carlia, except burnetti, lack fused lower eyelids and have a typically anvil shaped presubocular”, allocating timlowi to Menetia seemed the best available option. Ingram and Covacevich (1988) revisited the generic status of this species when resurrecting Lygisaurus from the synonymy of Carlia to accommodate a group of small leaf-litter dwelling skinks. In proposing the new combination L. timlowi (Ingram 1977), they considered the supraoculars to be transverse whereas Ingram had stated them to be oblique in his original species description

Single-Frequency GPS Relative Navigation in a High Ionosphere Orbital Environment

The Global Positioning System (GPS) provides a convenient source for space vehicle relative navigation measurements, especially for low Earth orbit formation flying and autonomous rendezvous mission concepts. For single-frequency GPS receivers, ionospheric path delay can be a significant error source if not properly mitigated. In particular, ionospheric effects are known to cause significant radial position error bias and add dramatically to relative state estimation error if the onboard navigation software does not force the use of measurements from common or shared GPS space vehicles. Results from GPS navigation simulations are presented for a pair of space vehicles flying in formation and using GPS pseudorange measurements to perform absolute and relative orbit determination. With careful measurement selection techniques relative state estimation accuracy to less than 20 cm with standard GPS pseudorange processing and less than 10 cm with single-differenced pseudorange processing is shown

Flexible Execution of Plans with Choice

Dynamic plan execution strategies allow an autonomous agent to respond to uncertainties while improving robustness and reducing the need for an overly conservative plan. Executives have improved this robustness by expanding the types of choices made dynamically, such as selecting alternate methods. However, in methods to date, these additional choices introduce substantial run-time latency. This paper presents a novel system called Drake that makes steps towards executing an expanded set of choices dynamically without significant latency. Drake frames a plan as a Disjunctive Temporal Problem and executes it with a fast dynamic scheduling algorithm. Prior work demonstrated an efficient technique for dynamic execution of one special type of DTPs by using an off-line compilation step to find the possible consistent choices and compactly record the differences between them. Drake extends this work to handle a more general set of choices by recording the minimal differences between the solutions which are required at run-time. On randomly generated structured plans with choice, we show a reduction in the size of the solution set of over two orders of magnitude, compared to prior art

Politics of Aedification, Sensation and Ruination at the Brussels Wiertz Museum

This paper seeks to explain why party candidates and their party leadership have congruent policy positions or not. Despite its importance as a way through which parties are able to behave as a unitary actor, this congruence has never been studied as a dependent variable. We seek to fill this void in the literature. Our results suggest that leadership-candidate congruence comes about through two mechanisms: selection and learning. With selection, the party leadership aims to get those candidates elected whose policy preferences are congruent with the party line. Learning occurs through the process of socialization in which candidates assume the views of the party they work and candidate for as their own under. This happens under the pressure of cognitive dissonance. If a candidate learns about the position of the leadership and notices that they are incongruent, they may feel discomfort and change their opinion to be congruent with the party