Konstantinos Adamidis

A lifetime distribution with decreasing failure rate

A two-parameter distribution with decreasing failure rate is introduced. Various properties are discussed and the estimation of parameters is studied by the method of maximum likelihood. The estimates are attained by the EM algorithm and expressions for their asymptotic variances and covariances are obtained. Numerical examples based on real data are presented.

Automated management of nutrient solutions based on target electrical conductivity, ph, and nutrient concentration ratios

Abstract An algorithm to automatically formulate the composition and prepare nutrient solutions for soilless cultures based on desired characteristics given as target values is proposed. To formulate the complete ionic composition of a nutrient solution, standard recommendations referring to the following solution characteristics should be available: (i) electrical conductivity (EC), (ii) pH, (iii) concentration ratios of macronutrients (meq basis), and (iv) micronutrient concentrations. Moreover, the mineral composition of the water used to prepare the nutrient solution should be taken into account. To develop the main steps of the algorithm, the total salt concentration (meq L‐1) of the nutrient solution is related to the desired electrical conductivity. The target pH indicates the concentration of HCO3 − in the nutrient solution, whereas the absolute concentration of each macronutrient is derived from the desired concentration ratios and the total salt concentration. The necessary calculations are stan...

A Lifetime Distribution With an Upside-Down Bathtub-Shaped Hazard Function

A three-parameter lifetime distribution with increasing, decreasing, bathtub, and upside down bathtub shaped failure rates is introduced. The new model includes the Weibull distribution as a special case. A motivation is given using a competing risks interpretation when restricting its parametric space. Various statistical properties, and reliability aspects are explored; and the estimation of parameters is studied using the standard maximum likelihood procedures. Applications of the model to real data are also included

Theory & Methods: An EM algorithm for estimating negative binomial parameters

An EM algorithm is proposed for computing estimates of parameters of the negative bi-nomial distribution; the algorithm does not involve further iterations in the M-step, in contrast with the one given in Schader & Schmid (1985). The approach can be applied to the corresponding problem in the logarithmic series distribution. The convergence of the proposed scheme is investigated by simulation, the observed Fisher information is derivedand numerical examples based on real data are presented.

A Family of Lifetime Distributions

A four-parameter family of Weibull distributions is introduced, as an example of a more general class created along the lines of Marshall and Olkin, 1997. Various properties of the distribution are explored and its usefulness in modelling real data is demonstrated using maximum likelihood estimates.

Bivariate Extended Exponential-Geometric Distributions

In this article four derivations are presented, for an absolutely continuous bivariate extension of the Extended Exponential-Geometric distribution (EEG) introduced by Adamidis et al. (2005). Three of these derivations are based on “shock models” and one is based on the assumption of a two component system working in a varying environment. Marginal and conditional distributions are obtained and their corresponding survival and hazard functions are calculated. The dependence in the proposed bivariate distributions is evaluated by means of the Pearson correlation coefficient.

Use of Prostaglandin E2 for Cervical Ripening in Pregnancies with Oligohydramnios

The effectiveness of intracervical prostaglandin E2 (PGE2) was studied in 96 primigravidas with unfavorable cervix and need for induction of labor. Group A consisted of 49 oligohydramnios and group B of 47 cases with normal amniotic fluid volume (controls). A single dose of 0.5 mg PGE2 gel was applied intracervically and several parameters were recorded during the next hours. The mean number of uterine contractions increased during the first 2 h in both groups, decreased during the next one and did not change significantly afterwards. The mean fetal heart rate (FHR) in group A decreased during the first 2 h and then increased, staying always within normal limits. The mean FHR in group B increased for 2 h, decreased during the next hour and did not change significantly afterwards. No significant differences were found between the two groups regarding mode of delivery, Apgar score and neonatal acidosis. Intracervical PGE2 appeared to effectively stimulate cervical ripening and labor induction in oligohydramnios, without causing side effects to the uterus and fetus.

Estimation in a truncated bivariate poisson distribution using the EM algorithm

An EM algorithm (Dempster et al., 1977) is derived for the estimation of parameters of the truncated bivariate Poisson distribution with zeros rnissing from both margins. The observed inforrnation matrix is obtained and a numerical exarnple is given where the convergence of the EM algorithm is accelerated by the methods of Louis (1982) and conjugate gradients (Jamshidian antl Jennrich, 1993).

Bivariate Distributions with Pearson Type VII Conditionals

In this article the most general class of bivariate distributions such that both conditional densities are Pearson Type VII, with fixed shape parameter, is fully characterized. Some of its properties and relations with other distributions are explored. The estimation of parameters is considered by the methods of maximum likelihood and pseudolikelihood and a method for random variate generation is presented along with a simulation experiment. Bivariate and multivariate extensions of the Pearson Type VII conditionals distribution are also discussed.

Distribution of Two X-Linked Trinucleotide Polymorphisms in Greece

Objective: To determine the distribution of allele frequencies of two X-linked trinucleotide repeat polymorphisms in five major geographic areas of Greece. Methods: We have studied the distribution of the CGG repeat in the FMR-1 gene and of the CAG repeat in the androgen receptor (AR) gene in 194 and 175 unrelated blood donors, respectively. Results: The distribution of (CGG)n was not significantly different between the four main areas of the Hellenic peninsula, with the exception of an island area in southern Greece, while the distribution of (CAG)n was similar among the groups. The mean repeat number in the FMR-1 gene is 30 and the mean in the AR gene is 21. Conclusions: The Hellenic population is rather homogeneous regarding the X-linked polymorphisms studied.