George Tzavelas

Increased sensitization in urban vs. rural environment--rural protection or an urban living effect?

In a population‐based longitudinal cohort study, we tested the hypothesis that children growing up in a high‐traffic polluted urban area (UA) in the Athens’ basin have higher prevalence of allergies and sensitization when compared with those growing up in a Greek provincial rural area (RA). We recruited 478 and 342 children aged 8–10 living in the UA and the RA, respectively. Respiratory health was assessed by a parent‐completed questionnaire in three phases: 1995–96 (phase 1), 1999–2000 (phase 2), 2003–04 (phase 3) and skin‐prick testing to common indoor and outdoor aeroallergens was performed at phases 1 and 2. Reported asthma and eczema did not differ between the two areas, whereas reported hay fever was persistently more prevalent in the UA than in the RA (16.5%, 17.0%, 18.2% vs. 7.0%, 8.3%, 9.6%, respectively). Sensitization was more prevalent in the UA at both phases (19.0% vs. 12.1% in phase 1, 20.0% vs. 14.1% in phase 2). Residential area contributed independently to sensitization to ≥1 aeroallergens (OR: 0.29; 95% CI: 0.13–0.66; p = 0.003) and to polysensitization (OR: 0.28; 95% CI: 0.10–0.82; p = 0.020) in phase 1. These associations were independent of farming practices. No significant contributions were found in phase 2. Our results suggest that long‐term exposure to urban environment is associated with a higher prevalence of hay fever but not of asthma or eczema. The negative association between rural living and the risk of atopy during childhood, which is independent of farming practices, implies that it is mainly driven by an urban living effect.

A note on the uniqueness of the quasi-likelihood estimator

The existence and the asymptotic performance of the quasi-likelihood estimator was studied by Kagan (1976), McCullagh (1983) and Morton (1981). However, their proof do not reveal whether multiple roots may yield one or more consistent solutions. In this paper, arguing as Cramer (1946), Huzurbazar (1948) we prove that if there is a consistent quasi-likelihood estimator, then it is unique with probability tending to one.

Validity of dietary patterns derived in nutrition surveys using a priori and a posteriori multivariate statistical methods.

Aim: To evaluate the validity of dietary patterns derived using both a priori and a posteriori methods.  Method: Five hundred individuals (46 ± 16 years, 40% males) completed a valid 76-food item food frequency questionnaire and a 3-day dairy. The MedDietScore was used to a priori assess the adherence to the Mediterranean diet, while principal components and cluster analysis were used as the a posteriori methods.  Results: Both the a priori and a posteriori methods used led to relatively low-validity dietary patterns. However, the level of validity reached significance in many cases and, also, varied by the type of validation method used (i.e. Bland and Altman, non-parametric criteria, etc.).  Conclusion: The aforementioned findings may suggest that the use of both a priori and a posteriori pattern analysis in nutrition surveys should be made with conscious thought and further research is needed in order to establish robust methodologies to assess the validity of patterns.

Sufficient Conditions for the Existence of a Solution for the Log-Likelihood Equations in the Three-Parameter Gamma Distribution

The existence of maximum likelihood estimators for the three-parameter gamma distribution is still an open problem. It demands the solution of the system of the log-likelihood equations which can be solved only with the use of numerical methods. The aim of this article is to provide sufficient conditions for the existence of a solution for the system of the log-likelihood equations. The conditions are expressed in terms of the geometric, arithmetic, and harmonic mean. Also, the importance of the sign of the third central moment for the existence of a solution is revealed.

Different effects of sensitization to mites and pollens on asthma symptoms and spirometric indices in children: a population-based cohort study

BACKGROUND We have previously shown that long-term exposure of children to a highly polluted urban compared with a rural environment is associated with subclinical airway narrowing and increased prevalence of atopy. OBJECTIVE To test the hypothesis that sensitization to indoor perennial, compared with seasonal, aeroallergens has distinct effects on asthma symptoms and/or spirometric indices. METHODS We evaluated the respiratory health of 478 and 342 children aged 8 to 10 years living in an urban and a rural area, respectively, during a period of 8 years. Children were evaluated by parental questionnaire in 3 phases, 1995 to 1996 (phase 1), 1999 to 2000 (phase 2), and 2003 to 2004 (phase 3), and by spirometry and skin prick testing to 9 common local aeroallergens in phases 1 and 2. RESULTS Sensitization to pollens was associated with current wheezing in phase 1 of the study (odds ratio [OR], 3.36; 95% confidence interval [CI], 1.71 to 6.62; P < .001) but not with spirometric indices. Sensitization to mites was negatively associated with forced expiratory volume in 1 second (95% CI, -7.26 to -0.90; P = .01) and forced expiratory flow at 50% of forced vital capacity (95% CI, -10.80 to -1.33; P = .01) in study phase 1 but not in phase 2. CONCLUSIONS Our results indicate that sensitization to mites is associated with insidious involvement of large and small airways, whereas sensitization to pollens is associated with childhood wheezing at the age of 8 to 10 years. Subsequent loss of these associations implies that risk factors other than allergy influence airway disease at a later age.

Repeatability of dietary patterns extracted through multivariate statistical methods: a literature review in methodological issues

Abstract In the last few years, the need for processing large amount of data in nutrition science was dramatically arose. This created the need to apply, primarily, advanced analytical research methods that could enable researchers to handle the large amount of information. Dietary pattern analysis is a commonly used approach to enable and incorporate this phenomenon in nutrition research. This article reviews the most common dietary pattern’s assessment statistical methods, evaluating at the same time the up-to-day knowledge regarding the reliability and validity of the retrieved patterns. The review is based on both a-priori (diet scores) and a-posteriori (multivariate statistical analysis) methods. The reports from the existing few studies suggest that the use of both a-priori and a-posteriori pattern analyses in nutrition surveys should be made with consciousness. The suggestion of new statistical techniques for the control of repeatability of dietary patterns is considered essential.

Statistical inference for the size-biased Weibull distribution

In this paper, statistical inferences for the size-biased Weibull distribution in two different cases are drawn. In the first case where the size r of the bias is considered known, it is proven that the maximum-likelihood estimators (MLEs) always exist. In the second case where the size r is considered as an unknown parameter, the estimating equations for the MLEs are presented and the Fisher information matrix is found. The estimation with the method of moments can be utilized in the case the MLEs do not exist. The advantage of treating r as an unknown parameter is that it allows us to perform tests concerning the existence of size-bias in the sample. Finally a program in Mathematica is written which provides all the statistical results from the procedures developed in this paper.

Maximum likelihood parameter estimation in the three-parameter gamma distribution with the use of Mathematica

The number of solutions of the system of the log-likelihood equations for the three-parameter case is still an open problem. Several methods have been developed for finding the solutions. In this article we present a program in Mathematica that can find all the solutions of the system of equations. Furthermore, we examine the case where the global maximum appears at the boundary of the domain of the log-likelihood function and we prove that any consistent estimators appear at the interior with probability tending to one.

Tests of fit for a lognormal distribution

The problem of goodness of fit of a lognormal distribution is usually reduced to testing goodness of fit of the logarithmic data to a normal distribution. In this paper, new goodness-of-fit tests for a lognormal distribution are proposed. The new procedures make use of a characterization property of the lognormal distribution which states that the Kullback–Leibler measure of divergence between a probability density function (p.d.f) and its r-size weighted p.d.f is symmetric only for the lognormal distribution [Tzavelas G, Economou P. Characterization properties of the log-normal distribution obtained with the help of divergence measures. Stat Probab Lett. 2012;82(10):1837–1840]. A simulation study examines the performance of the new procedures in comparison with existing goodness-of-fit tests for the lognormal distribution. Finally, two well-known data sets are used to illustrate the methods developed.

Sample Tests for Detection of Size-Biased Sampling Mechanism

The aim of this article is to present a new test for the detection of size-bias in a sample with or without censored observations. The test is simple in the form and demands only the knowledge of consistent estimators of any nuisance parameters appeared in the model. With the use of simulated samples from the Weibull distribution, we show the advantages of the new test compared to the Likelihood Ratio and the Wald test.