On the use of a Modified Latin Hypercube Sampling (MLHS) approach in the estimation of a Mixed Logit model for vehicle choice

Quasi-random number sequences have been used extensively for many years in the simulation of integrals that do not have a closed-form expression, such as Mixed Logit and Multinomial Probit choice probabilities. Halton sequences are one example of such quasi-random number sequences, and various types of Halton sequences, including standard, scrambled, and shuffled versions, have been proposed and tested in the context of travel demand modeling. In this paper, we propose an alternative to Halton sequences, based on an adapted version of Latin Hypercube Sampling. These alternative sequences, like scrambled and shuffled Halton sequences, avoid the undesirable correlation patterns that arise in standard Halton sequences. However, they are easier to create than scrambled or shuffled Halton sequences. They also provide more uniform coverage in each dimension than any of the Halton sequences. A detailed analysis, using a 16-dimensional Mixed Logit model for choice between alternative-fuelled vehicles in California, was conducted to compare the performance of the different types of draws. The analysis shows that, in this application, the Modified Latin Hypercube Sampling (MLHS) outperforms each type of Halton sequence. This greater accuracy combined with the greater simplicity make the MLHS method an appealing approach for simulation of travel demand models and simulation-based models in general

On a Tree and a Path with no Geometric Simultaneous Embedding

Two graphs $G_1=(V,E_1)$ and $G_2=(V,E_2)$ admit a geometric simultaneous embedding if there exists a set of points P and a bijection M: P -> V that induce planar straight-line embeddings both for $G_1$ and for $G_2$. While it is known that two caterpillars always admit a geometric simultaneous embedding and that two trees not always admit one, the question about a tree and a path is still open and is often regarded as the most prominent open problem in this area. We answer this question in the negative by providing a counterexample. Additionally, since the counterexample uses disjoint edge sets for the two graphs, we also negatively answer another open question, that is, whether it is possible to simultaneously embed two edge-disjoint trees. As a final result, we study the same problem when some constraints on the tree are imposed. Namely, we show that a tree of depth 2 and a path always admit a geometric simultaneous embedding. In fact, such a strong constraint is not so far from closing the gap with the instances not admitting any solution, as the tree used in our counterexample has depth 4.Comment: 42 pages, 33 figure

Teleradiology usage and user satisfaction with the telemedicine system operated by Médecins Sans Frontières

Médecins Sans Frontières (MSF) began a pilot trial of store-and-forward telemedicine in 2010, initially operating separate networks in English, French, and Spanish; these were merged into a single, multilingual platform in 2013. We reviewed the pattern of teleradiology usage on the MSF telemedicine platform in the 4-year period from April 2010. In total, 564 teleradiology cases were submitted from 22 different countries. A total of 1114 files were uploaded with the 564 cases, the majority being of type JPEG (n = 1081, 97%). The median file size was 938 kb (interquartile range, IQR 163-1659). A panel of 14 radiologists was available to report cases, but most (90%) were reported by only 4 radiologists. The median radiologist response time was 6.1 h (IQR 3.0-20). A user satisfaction survey was sent to 29 users in the last 6 months of the study. There was a 28% response rate. Most respondents found the radiologist's advice helpful and all of them stated that the advice assisted in clarification of a diagnosis. Although some MSF sites made substantial use of the system for teleradiology, there is considerable potential for expansion. More promotion of telemedicine may be needed at different levels of the organization to increase engagement of staff

Evaluation of a chest radiograph reading and recording system for tuberculosis in a HIV-positive cohort.

Aim To assess the impact of introducing a chest radiograph reading and recording system (CRRS) with a short training session, on the accuracy and inter-reader variability of tuberculosis (TB) interpretation of chest radiographs (CXRs) by a group of non-expert readers in a human immunodeficiency virus (HIV)-positive cohort. Materials and methods A set of 139 CXRs was reviewed by a group of eight physicians pre- and post-intervention at two clinics in Shan State, Myanmar, providing HIV/TB diagnosis and treatment services. The results were compared against the consensus of expert radiologists for accuracy. Results Overall accuracy was similar pre- and post-intervention for most physicians with an average area under the receiver operating characteristic curve difference of 0.02 (95% confidence interval: –0.03, 0.07). The overall agreement among physicians was poor pre- and post-intervention (Fleiss κ=0.35 and κ=0.29 respectively). The assessment of agreement for specific disease patterns associated with active TB in HIV-infected patients showed that for intrinsically subtle findings, the agreement was generally poor but better for the more intrinsically obvious disease patterns: pleural effusion (Cohen’s kappa range = 0.37–0.67) and milliary nodular pattern (Cohen’s kappa range = 0.25–0.52). Conclusion This study demonstrated limited impact of the introduction of a CRRS on CXR accuracy and agreement amongst non-expert readers. The role in which CXRs are used for TB diagnosis in a HIV-positive cohort in similar clinical contexts should be reviewed

An alternative procedure for selecting a good value for the parameter c in RBF-interpolation

The impact of the scaling parameter c on the accuracy of interpolation schemes using radial basis functions (RBFs) has been pointed out by several authors. Rippa (Adv Comput Math 11:193-210, 1999) proposes an algorithm based on the idea of cross validation for selecting a good such parameter value. In this paper we present an alternative procedure, that can be interpreted as a refinement of Rippa's algorithm for a cost function based on the euclidean norm. We point out how this method is related to the procedure of maximum likelihood estimation, which is used for identifying covariance parameters of stochastic processes in spatial statistics. Using the same test functions as Rippa we show that our algorithm compares favorably with cross validation in many cases and discuss its limitations. Finally we present some computational aspects of our algorithm

Assessment of scattered and leakage radiation from ultra-portable digital chest X-ray systems: An independent study

Ultraportable X-ray devices are ideal for TB screening in resource-limited settings. Unfortunately, guidelines on the radiation safety of these devices are lacking. The aim of the study was to determine the radiation dose by scattered and leakage radiation of ultraportable X-ray devices to provide a basis for these guidelines. Radiation dose measurements were performed with four ultraportable X-ray devices that meet the WHO-IAEA criteria. An anthropomorphic thorax phantom was positioned in posterior-anterior orientation in a clinically representative X-ray setup. X-ray exams were acquired with the following scan parameters: 90kV, 2.5mAs and maximum mAs, 1m and 1.8m source-to-skin distance. The entrance skin dose was measured at the center of the phantom. The scattered radiation dose was measured at 1m from the phantom as a function of scatter angle. Leakage radiation was measured at 0.5m from the X-ray tube with collimators closed and covered with additional lead. From the scatter measurements, the 'safe distance bunny' was determined, which was the minimum distance to the phantom to stay below the international dose limit to the public (1mSv per year) at a given workload. The longest distance (related to highest scatter dose) was observed behind the edges of the detector and back towards the X-ray tube, whereas the shortest distance (related to lowest scatter dose) was observed to the sides of the phantom. For the radiographer position, the total radiation dose by scattered and leakage radiation was determined in various scenarios. In most cases, the total radiation dose of ultraportable X-ray devices can be kept below 1mSv per year by employing basic radiation safety rules:1. Reduce time in the field, 2. Increase distance to the X-ray source, and 3. Use shielding measures (lead apron). Ultraportable X-ray devices can be safely used for TB screening when using adequate precautions

Reducing Calorie Intake May Not Help You Lose Body Weight

Background Previously a meta-analysis found that multi-vitamin/mineral supplementation reduced mild psychiatric symptoms. To establish mechanisms, and to pin-point the individuals most likely to benefit, the role of various polymorphisms were examined. Supplementation was found to influence mild-psychiatric symptoms depending on the form of particular genes: genes that are risk factors for psychiatric disease and influence mechanisms by which drugs act. Methods In a double-blind trial young healthy males rated psychiatric symptoms, before and after taking vitamin/mineral supplements for three months, and the response was related to single nucleotide polymorphisms associated with catecholamines and serotonin. Outcomes With rs1800497 (Taq1A; dopamine D2 receptor), those with the CT allele benefitted from a vitamin/mineral supplement. Similarly with rs1800955 (DRD4 – dopamine D4 receptor), the mood of those with the CC allele benefitted selectively. With rs6296 (HTR1B) only those with the GC alleles responded, and with rs6311 (HTR2A) supplementation produced a beneficial response in those with the GG allele. With rs1050565 (5HTT gene - Human Serotonin Transporter gene) supplementation increased the mental health of those with the AA allele. Interpretation In a situation where a substantial proportion of patients do not benefit from drug therapy, and there is an element of trial and error when prescribing, it was proposed that future work should consider distinguishing patients depending on various polymorphisms and micro-nutrient status. In those with particular alleles, we should consider if drug administration and vitamin / mineral status interact synergistically to influence the therapeutic outcom

Dietary factors impact on the association between CTSS variants and obesity related traits.

Cathepsin S, a protein coded by the CTSS gene, is implicated in adipose tissue biology--this protein enhances adipose tissue development. Our hypothesis is that common variants in CTSS play a role in body weight regulation and in the development of obesity and that these effects are influenced by dietary factors--increased by high protein, glycemic index and energy diets

QSOs Associated with Messier 82

The starburst / AGN galaxy M82 was studied by Dahlem, Weaver and Heckman using X-ray data from ROSAT and ASCA, as part of their X-ray survey of edge-on starburst galaxies. They found seventeen unresolved hard-X-ray sources around M82, in addition to its strong nuclear source, and other X-rays within the main body of M82. We have measured optical point sources at these positions, and have obtained redshifts of six candidates at the Keck I 10-m telescope, using the low-resolution imaging spectrograph (LRIS). All six are highly compact optical and X-ray objects with redshifts ranging from 0.111 to 1.086. They all show emission lines. The three with the highest redshifts are clearly QSOs. The others with lower redshifts may either be QSOs or compact emission-line galaxies. In addition to these six there are nine QSOs lying very close to M82 which were discovered many years ago. There is no difference between optical spectra of these latter QSOs, only two of which are known to be X-ray sources, and the X-ray emitting QSOs. The redshifts of all fifteen range between 0.111 and 2.05. The large number of QSOs and their apparent association with ejected matter from M82 suggest that they are physically associated with the galaxy, and have large intrinsic redshift components. If this is correct, the absolute magnitudes lie in the range -8 < M_v < -10. Also we speculate that the luminous variable X-ray source which has been detected by Chandra in the main body of M82 some 9 arcseconds from the center is another QSO in the process of ejection from the nucleus, and propose some observational tests of this hypothesis.Comment: 16 pages aastex, 3 eps figures, accepted for publication on Ap