Reducing variance in univariate smoothing

A variance reduction technique in nonparametric smoothing is proposed: at each point of estimation, form a linear combination of a preliminary estimator evaluated at nearby points with the coefficients specified so that the asymptotic bias remains unchanged. The nearby points are chosen to maximize the variance reduction. We study in detail the case of univariate local linear regression. While the new estimator retains many advantages of the local linear estimator, it has appealing asymptotic relative efficiencies. Bandwidth selection rules are available by a simple constant factor adjustment of those for local linear estimation. A simulation study indicates that the finite sample relative efficiency often matches the asymptotic relative efficiency for moderate sample sizes. This technique is very general and has a wide range of applications.Comment: Published at http://dx.doi.org/10.1214/009053606000001398 in the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org

A nonparametric procedure for testing partially ranked data

[[abstract]]In consumer preference studies, it is common to seek a complete ranking of a variety of, say N, alternatives or treatments. Unfortunately, as N increases, it becomes progressively more confusing and undesirable for respondents to rank all N alternatives simultaneously. Moreover, the investigators may only be interested in consumers’ top few choices. Therefore, it is desirable to accommodate the setting where each survey respondent ranks only her/his most preferred k (k < N) alternatives. In this paper, we propose a simple procedure to test the independence of N alternatives and the top-k ranks, such that the value of k can be predetermined before securing a set of partially ranked data or be at the discretion of the investigator in the presence of complete ranking data. The asymptotic distribution of the proposed test under root-n local alternatives is established. We demonstrate our procedure with two real data sets.[[sponsorship]]科技部[[notice]]補正完

A note on kernel polygons

Jones (1989) has pointed out that piecewise linear interpolated kernel density estimators on a sufficiently fine grid can be visually indistinguishable from the true density. A simple device, the kernel polygon, is proposed for eliminating the evaluation of the normalisation constant of the estimator while retaining its property of being a density function as well as providing practical advantages. The class of uniform and linear kernels of the kernel polygons is given. Finally, we present a simulation study and a real data example in which we compare bandwidth selectors for the kernel polygons. Copyright 2006, Oxford University Press.

Note on Changing-Point Testing for Nonparametric Regression Function

[[abstract]]Nonparametric regression analysis is an important and popular method to investigate the regression model of the source of data. Traditionally, regression analysis is often used under the assumption that the regression function is continuous and smooth. However in a real world situation, it is quite common that due to the presence of an external force, the regression function has change points. Because of the boundary effect of change points, the estimated regression function derived by nonparametric regression method may be far from the real model. Since the kernel estimator of a nonparametric regression function is simple and has nice asymptotic properties, it has been widely used in estimating the regression function. However, apart from being dependent on the choice of a smoothing parameter, these nice properties depend on the continuity of the regression function. From recent researches, we have learned that the ignorance of the possible existence of change points will lose the nice properties of the kernel estimator of a regression function. Therefore, it is very important to decide whether a change point is present somewhere. In this article, we shall use the L-2 norms of the DKE (difference between two kernel estimators) which are proposed in Wu and Chu (1993) to construct a new estimator to test whether the change point exists or not. We shall also derive the limiting distribution of this test estimator

Consensus of gastroesophageal reflux disease in Taiwan with endoscopy-based approach covered by National Health Insurance

Background and aims: Gastroesophageal reflux disease (GERD) is emerging as a clinical complication in the Orient. The consensus comprises recommendations to GERD control under the advantage of endoscopy-based approach covered by the Taiwan National Health Insurance. Methods: The steering committee defined the consensus scope to cover diagnostic, therapeutic, unresolved, controversial, or long-term proton pump inhibitor-related issues to GERD. The literature review emphasized domestic data, after which the draft statements and statement evidence levels were defined. Thirty-five experts of GERD in Taiwan formed the expert group to conduct the consensus conference by a modified Delphi process to vote anonymously to reach a consensus, defined by an agreement of ≥ 80% for each statement, and to set the recommendation grade. Results: The consensus included 22 statements, including seven on diagnostic approach, seven on therapeutic suggestion, and eight on unresolved, controversial, or long-term proton pump inhibitor-related issues to GERD. The consensus highlighted that the endoscopy approach to GERD can define the disease spectrum and exclude malignant potential. The questionnaire survey can not only define GERD, but also monitor treatment response and quality of life. The consensus addressed suggestions for the unresolved issues related to extraesophageal presentation and adverse concerns of GERD after long-term use of proton pump inhibitors. In the endemic area of upper gastrointestinal cancers, Helicobacter pylori eradication is suggested to reduce progression of gastric precancerous lesions, and endoscopic surveillance of Barrett’s esophagus with dysplasia deserves prospective research. Conclusion: The consensus comprises recommendations for the management of GERD in a high upper gastrointestinal cancer area with a national coverage of endoscopic approach

Survey of the adherence to the consensus of gastroesophageal reflux disease before and after the implementation course

Background/Purpose: The prevalence of Gastroesophageal reflux disease (GERD) is increasing worldwide, including Asia. Although several consensus reports have been published, little is known regarding the adherence of the physicians on the consensus of GERD. We aimed to survey the agreements and adherence of physicians to the Taiwan GERD consensus before and after the continual medical education (CME) courses. Methods: Two-hundred and twenty-seven physicians, including 81 fellows of gastroenterology, 135 qualified gastroenterologists, and 11 non-gastroenterologist attending physicians were invited to the CME course. Their agreements and adherence to the statements before and after the CME course were assessed by the pre-defined questionnaire with the aid of electronic keypads. The adherence rate before and after the CME course were compared by the McNemar test to indicate the changes in their willingness to follow the statements in clinical practice. Results: The rates of agreement of the 227 participating physicians were uniformly greater than 80% for all of the 22 statements. However, the adherence rates were lower than 80% in 16 statements before the CME intervention. The adherence rates were significantly (p < 0.05) increased in 15 of these 16 statements after the CME intervention. The adherence rate can be improved to greater than 80% for those statements with high level of evidence. Conclusion: Although physicians agreed with the statements, the pre-CME survey disclosed limited adherence rates to the statements. The education intervention through the CME courses can improve the adherence of consensus statement, especially for those with higher level of evidence. Keywords: Adherence, Consensus, Barrier, Gastroesophageal reflux disease, Educatio

Averaged shifted chi-square test

[[abstract]]A simple procedure based on the average of shifted chi-square statistics (ASCS) is proposed to improve the classical chi-square procedure for testing whether a random sample has been drawn from a specified continuous distribution. We repeatedly partition the sample space, say, ℓ times to obtain ℓ respective chi-square statistics. The proposed test statistic is defined as the average value of the resultant ℓ shifted chi-square statistics. We prove that the ASCS is asymptotically distributed as a weighted sum of a finite number of chi-square variables by the theory of U-statistics. The proposed procedure is shown to be markedly less sensitive to the choice of the anchor position and Monte Carlo experiments demonstrate that it leads to noticeable gains in power.[[notice]]補正完畢[[journaltype]]國外[[incitationindex]]SCI[[booktype]]紙本[[countrycodes]]GB

The Primary Resistance of Helicobacter pylori in Taiwan after the National Policy to Restrict Antibiotic Consumption and Its Relation to Virulence Factors-A Nationwide Study.

The Taiwan Government issued a policy to restrict antimicrobial usage since 2001. We aimed to assess the changes in the antibiotic consumption and the primary resistance of H. pylori after this policy and the impact of virulence factors on resistance.The defined daily dose (DDD) of antibiotics was analyzed using the Taiwan National Health Insurance (NHI) research database. H. pylori strains isolated from treatment naïve (N=1395) and failure from prior eradication therapies (N=360) from 9 hospitals between 2000 and 2012 were used for analysis. The minimum inhibitory concentration was determined by agar dilution test. Genotyping for CagA and VacA was determined by PCR method.The DDD per 1000 persons per day of macrolides reduced from 1.12 in 1997 to 0.19 in 2008, whereas that of fluoroquinolones increased from 0.12 in 1997 to 0.35 in 2008. The primary resistance of amoxicillin, clarithromycin, metronidazole, and tetracycline remained as low as 2.2%, 7.9%, 23.7%, and 1.9% respectively. However, the primary levofloxacin resistance rose from 4.9% in 2000-2007 to 8.3% in 2008-2010 and 13.4% in 2011-2012 (p=0.001). The primary resistance of metronidazole was higher in females than males (33.1% vs. 18.8%, p<0.001), which was probably attributed to the higher consumption of nitroimidazole. Neither CagA nor VacA was associated with antibiotic resistance.The low primary clarithromycin and metronidazole resistance of H. pylori in Taiwan might be attributed to the reduced consumption of macrolides and nitroimidazole after the national policy to restrict antimicrobial usage. Yet, further strategies are needed to restrict the consumption of fluoroquinolones in the face of rising levofloxacin resistance