Maximal Reliability Research Articles

Network reliability assessment using a cellular automata approach

Abstract Two cellular automata (CA) models that evaluate the s–t connectedness and shortest path in a network are presented. CA based algorithms enhance the performance of classical algorithms, since they allow a more reliable and straightforward parallel implementation resulting in a dynamic network evaluation, where changes in the connectivity and/or link costs can readily be incorporated avoiding recalculation from scratch. The paper also demonstrates how these algorithms can be applied for network reliability evaluation (based on Monte-Carlo approach) and for finding s–t path with maximal reliability.

A recursively heuristic method for the reliability optimization of a distributed computing system

The reliability optimization of a distributed computing system is generally an NPhard problem. The reliability of a k–node set with capacity constraint is defined as the probability that a subset of the set of processing elements in system is connected and possesses sufficient node capacity. Relative investigations, namely an exact method and a k–tree reduction method, have examined an optimal or nearly optimal k-node set with capacity constraint. Such investigations can not effectively reduce either the computational time complexity or the deviation from an exact solution. The paper presents a recursively heuristic method to overcome these difficulties. The proposed algorithm selects reasonable k–node sets by starting at two nodes which are incident with a link of maximal reliability and a node which has either maximal capacity or maximal degree, respectively. When the starting node is taken, the recursive sub-procedure is run. The sub-procedure adds an adjacent node which connected by a link of maximal reliability to selected set and calls itself again and again until an adequate k–node set is found. By doing so, the sub-procedure finds all of the adequate k–node sets and returns an optimal k–node set to the algorithm. In addition, the sub-procedure omits the reliability computation when an adequate set that has occurred before is selected. For the reason that the reliability of a subset of a set is at least as large as the reliability of the set, the reduction processing is used for decreasing the order of the set. The number of selected sets is very small, therefore the proposed algorithm can reduce execution time and complexity. Moreover, the deviation from the exact solution is improved. Compared with existing algorithms on various distributed computing system topologies, our results demonstrate that the proposed algorithm is more efficient in execution time than conventional methods.

On robusiness of the normal-theory based asymptotic distributions of three reliability coefficient estimates

This paper studies the asymptotic distributions of three reliability coefficient estimates: Sample coefficient alpha, the reliability estimate of a composite score following a factor analysis, and the estimate of the maximal reliability of a linear combination of item scores following a factor analysis. Results indicate that the asymptotic distribution for each of the coefficient estimates, obtained based on a normal sampling distribution, is still valid within a large class of nonnormal distributions. Therefore, a formula for calculating the standard error of the sample coefficient alpha, recently obtained by van Zyl, Neudecker and Nel, applies to other reliability coefficients and can still be used even with skewed and kurtotic data such as are typical in the social and behavioral sciences.

Optimal series–parallel topology of multi-state system with two failure modes

Abstract When considering systems built from units with two failure modes, changes in system topology that increases system reliability in open mode can decrease system reliability in closed mode and vice versa. For the given set of units composing a system there always exists a configuration which provides maximal reliability for the system experiencing both failure modes. In this paper, we suggest an algorithm for finding optimal series–parallel topology (configuration) for systems consisting of units characterized by different reliability and nominal performance rates. Such systems are multi-state because they can have different levels of output performance depending on the combination of units available at the moment. Reliability is defined as the probability of satisfaction of given constraints imposed on system performance in both modes. The procedure developed to solve this problem is based on the use of a universal moment generating function (UMGF) for the fast evaluation of multi-state system reliability and a genetic algorithm (GA) for optimization. Basic UMGF technique operators are developed for two different types of systems based, respectively, on transmitting capacity and processing time. Solution encoding for GA implementation is developed and basic GA procedures and parameters are chosen for the given problem. Examples of the optimization of series–parallel configurations for systems of both types are presented.

OPTIMAL ALLOCATION OF MULTISTATE ELEMENTS IN LINEAR CONSECUTIVELY-CONNECTED SYSTEMS WITH DELAYS

A linear consecutively-connected system consists of N + 2 linear ordered positions. The first position contains a source of a signal and the last one contains a receiver. M statistically independent multistate elements (retransmitters) with different characteristics are to be allocated at the N intermediate positions. The elements provide retransmission of the received signal to the next few positions. Each element can have different states determined by a number of positions that are reached by the signal generated by this element. The probability of each state for any given element depends on the position where it is allocated. The signal retransmission process is associated with delays. The system fails if the signal generated by the source can not reach the receiver within a specified time period. A problem of finding an allocation of the multistate elements that provides the maximal system reliability is formulated. An algorithm based on the universal generating function method is suggested for the system reliability determination. This algorithm can handle cases where any number of multistate elements are allocated in the same position while some positions remain empty. It is shown that such an uneven allocation can provide greater system reliability than an even one. A genetic algorithm is used as an optimization tool in order to solve the optimal element allocation problem.

Neuroanatomical substrata of amusement and sadness: a PET activation study using film stimuli.

The aim of the present study was to explore the neuroanatomical substrata of film-induced amusement and sadness, aiming at maximal reliability by using strict statistics. Film-induced changes in regional cerebral blood flow (rCBF) were measured using positron emission tomography and [15O]water. It appeared that many brain regions were involved similarly and symmetrically in both emotions. Activation of the occipitotemporal and anterior temporal cortex and cerebellum during both emotions agreed well with the earlier findings. Contrary to some earlier studies, the posterior cingulate gyrus and some frontal areas were deactivated in both emotions. The strict explorative analysis did not reveal activations in the limbic or subcorticai structures seen in some earlier studies. However, an additional analysis restricted to the subcortical and limbic system structures revealed bilateral activation of the amygdala in both target emotions. The outcomes of imaginal studies of emotions seem to be largely dependent on the conservativeness of the statistical analysis and very likely also on the method of emotion induction.

The Present Status of Maintenance Strategies and the Impact of Maintenance on Reliability

In this paper, the most frequently used power system maintenance strategies are reviewed. Distinction is made between strategies where maintenance consists of replacement by a new (or good new) component and where it is represented by a less costly activity resulting in a limited improvement of the component's condition. Methods are also divided into categories where maintenance is performed at fixed intervals and where it is carried out needed. A further distinction is made between heuristic methods and those based on mathematical models; the models themselves can be deterministic or probabilistic. From a review of present maintenance policies in electric utilities, it is concluded that maintenance at fixed intervals is the most frequently used approach, often augmented by additional corrections. Newer as needed-type methods, such reliability-centered maintenance (RCM), are increasingly considered for application in North America, but methods based on mathematical models are hardly ever used or even considered. Yet only mathematical approaches where component deterioration and condition improvement by maintenance are quantitatively linked can determine the effect of maintenance on reliability. Although more complex, probabilistic models have advantages over deterministic ones: they are capable of describing actual processes more realistically, and also facilitate optimization for maximal reliability or minimal costs.

The Present Status of Maintenance Strategies and the Impact of Maintenance on Reliability

In this paper, the most frequently used maintenance strategies are reviewed. Distinction is made between strategies where maintenance consists of replacement by a new (or "good as new") component and where it is represented by a less costly activity resulting in a limited improvement of the component's condition. Methods are also divided into categories where maintenance is performed at fixed intervals and where it is carried out as needed. A further distinction is made between heuristic methods and those based on mathematical models; the models themselves can be deterministic or probabilistic. From a review of present maintenance policies in electric utilities it is concluded that maintenance at fixed intervals is the most frequently used approach, often augmented by additional corrections. Newer "as needed"-type methods, such as reliability-centered maintenance (RCM), are increasingly considered for application in North America, but methods based on mathematical models are hardly ever used or even considered. Yet only mathematical approaches where component deterioration and condition improvement by maintenance are quantitatively linked can determine the effect of maintenance on reliability. Although more complex, probabilistic models have advantages over deterministic ones, they are capable of describing actual processes more realistically, and also facilitate optimization for maximal reliability or minimal costs.

Coefficient alpha and related internal consistency reliability coefficients.

The author studied the conditions under which coefficient alpha and 10 related internal consistency reliability coefficients underestimate the reliability of a measure. Simulated data showed that alpha, though reasonably robust when computed on n components in moderately heterogeneous data, can under certain conditions seriously underestimate the reliability of a measure. Consequently, alpha, when used in corrections for attenuation, can result in nontrivial overestimation of the corrected correlation. Most of the coefficients studied, including lambda 2, did not improve the estimate to any great extent when the data were heterogeneous. The exceptions were stratified alpha and maximal reliability, which performed well when the components were grouped into two subsets, each measuring a different factor, and maximized lambda 4, which provided the most consistently accurate estimate of the reliability in all simulations studied.

Beyond the Spearman-Brown: a structural approach to maximal reliability.

The requirement of parallel parts has long been the cornerstone of classic reliability theory. By recasting reliability in a structural equation framework, items, raters, or judges no longer need to be treated as equivalent entities. Instead, unique reliability estimates can be determined for each and collectively used to assess the maximal reliability of a weighted composite, with the composite reliability submitted to inferential test. Procedures are shown to generalize from single to multifactor applications. Ramifications of a structural approach to reliability determination are probed, and the dilemma posed by possible falsification of the true score hypothesis presented for individual researcher consideration.

Beyond the Spearman-Brown: A structural approach to maximal reliability.

Beyond the Spearman-Brown: A structural approach to maximal reliability.

Comparative reliability and validity of chronic pain intensity measures

Reliable and valid measures of pain are essential for conducting research on chronic pain. The purpose of this longitudinal study was to compare the reliability and validity of several measures of pain intensity. One hundred twenty-three patients with chronic pain were administered telephone interview versions of 0–10 scales of current, worst, least and average pain, immediately prior to beginning a multidisciplinary treatment program. The measures were administered again to these subjects 2 weeks ( n=108), 1 month ( n=106) and 2 months ( n=105) after the end of treatment. The validity (defined as ability to detect changes in pain intensity over the course of treatment up to the 2-month follow-up assessment) and reliability (defined as stability over time in the 2 months after treatment) of these four measures and of composite combinations of these measures were examined. Contrary to prediction, the composite measures did not show a statistically significant superiority to the individual ratings in terms of their ability to detect change in pain intensity from pre-treatment to various points after treatment. The composite scores did, however, show greater stability than did the individual ratings after treatment. The practical conclusions of this study are; (1), individual 0–10 pain intensity ratings have sufficient psychometric strengths to be used in chronic pain research, especially research that involves group comparison designs with relatively large sample sizes, but, (2), composites of 0–10 ratings may be more useful when maximal reliability is necessary, (e.g. in studies with relatively small sample sizes, or in clinical settings where monitoring of changes in pain intensity in individuals is needed).

Profiling steroid hormones in amniotic fluid of midpregnancy by routine stable isotope dilution/gas chromatography-mass spectrometry: reference values and concentrations in fetuses at risk for 21-hydroxylase deficiency.

Using routine stable isotope dilution/gas chromatography-mass spectrometry, 17-hydroxyprogesterone, androstenedione, testosterone, dehydroepiandrosterone, androstanediol, and 5alpha-dihydrotestosterone have been profiled in amniotic fluid of midgestation in 77 normal fetuses and 38 untreated or dexamethasone-treated fetuses at risk for 21-hydroxylase deficiency. Dexamethasone was suspended 5-7 days before amniocentesis. In normal fetuses, amniotic fluid concentrations (median, range; nanograms per mL) of 17-hydroxyprogesterone did not reveal a sex difference (1.48, 0.21-4.96), whereas those of androstenedione were lower in females (0.53, 0.00-2.71) than in males (0.93, 0.29-1.98). Testosterone levels were higher in males (0.24, 0.00-0.50) than in females (0.00, 0.00-0.27). No sex difference was found for dehydroepiandrosterone (0.47, 0.19-1.77). Levels of androstanediol and 5alpha-dihydrotestosterone were below the detection limit of our method in most cases. Regarding prenatal diagnosis of 21-hydroxylase deficiency, 17-hydroxyprogesterone and androstenedione presented the diagnostically most valuable steroids and were of equal diagnostic potential. They permitted successful diagnosis in 36 of 37 fetuses at risk: 12 were untreated and unaffected, 13 were treated and unaffected, 4 were untreated and affected (3 salt wasters and 1 simple virilizer), and 8 were treated and affected (5 salt wasters and 3 simple virilizers). In the latter group, one simple virilizer revealed normal steroid concentrations. Isotope dilution/gas chromatography-mass spectrometry, providing the highest specificity in steroid analysis, is proposed for routine use in clinical steroid analysis whenever maximal reliability is requested. Our study provides the first mass spectrometric reference data on amniotic fluid steroid concentrations and underscores the high accuracy of prenatal hormonal diagnosis of 21-hydroxylase deficiency.

Increasing Sampling Density Improves Reproducibility of Optical Coherence Tomography Measurements

Published series of peripapillary retinal nerve fiber layer (RNFL) measurements using optical coherence tomography (OCT) have sampled 100 evenly distributed points on a 360 degrees peripapillary circular scan. The goal of this study was to determine whether a four-fold increase in sampling density improves the reproducibility of OCT measurements. Complete ophthalmic examinations, achromatic automated perimetry, and OCT imaging were performed in all patients. The OCT scanning consisted of three superior and inferior quadrantic scans (100 sampling points/quadrant) and three circular scans (25 points/quadrant). The RNFL thickness measurements and coefficient of variation (CV) were calculated for the superior and inferior quadrants for each sampling density technique. The study included 22 eyes of 22 patients (3 control subjects; 2 patients with ocular hypertension; and 17 patients with glaucoma). Quadrants with associated glaucomatous visual field loss on automated achromatic perimetry had thinner RNFLs than quadrants without functional defects for both the 25- and 100-points/quadrant scans. For quadrants associated with normal visual hemifields (n = 22), there was no difference between the 25- and 100-points/quadrant scans in mean RNFL thickness and CV. Among quadrants with visual field defects (n = 22), RNFL thickness measurements were thinner in the 25-points/quadrant scans than in the 100-points/quadrant scans. The CV for the 25-points/quadrant scans (25.9%) was significantly higher than that for the 100-points/quadrant scans (11.9%). Increasing the sampling density of OCT scans provides less variable representation of RNFL thickness. The optimal sampling density to achieve maximal reliability of OCT scans remains to be determined.

A unifying expression for the maximal reliability of a linear composite

A formally simple expression for the maximal reliability of a linear composite is provided, its theoretical implications and its relation to existing results for reliability are discussed.

Reliability of measurement in psychology: From Spearman-Brown to maximal reliability.

Reliability of measurement in psychology: From Spearman-Brown to maximal reliability.

Reliability of measurement in psychology: From Spearman-Brown to maximal reliability.

The authors extend the usual approach to the assessment of test or rater reliability to situations that have previously not been appropriate for the application of this standard (Spearman-Brown) approach. Specifically, the authors (a) provide an accurate overall estimate of the reliability of a test (or a panel of raters) comprising 2 or more different kinds of items (or raters), a quite common situation, and (b) provide a simple procedure for constructing the overall instrument when it comprises 2 or more kinds of items, judges, or raters, each with its own costs and its own reliabilities.

Minimal size of the system with two dual modes of failure having prescribed reliability

Systems composed of independent identical units which can be in two failure modes - open and short - are considered. Among all monotone systems composed of n units, a k-out-of-n:G system has the maximal reliability - see [11]. In this paper, an estimate of the minimal size of a monotone system whose reliability exceeds 1 - ε is introduced. It is based on an investigation of the rate of convergence of reliabilities of the optimal systems with n units as n→∞.

Fundal height measurement: Part 2—Infra- and interexaminer reliability of three measurement techniques

The intra-and interexaminer reliability of three fundal height measurement techniques (two tape measure and one caliper) was studied in a convenience sample of 60 pregnant women. The three measurements were obtained twice by two examiners. Four examiners with varying levels of clinical experience participated in the study; one examiner obtained measurements in all 60 cases and the other three examiners each obtained measurements in approximately one-third of the cases. Examiners were blinded to their own and to other examiners' measurements. The mean absolute differences between individual examiner's first and second measurements varied from 0.68 to 1.74 cm, the percentage of differences ≤ 1.0 cm varied from 45% to 77.8%, and the maximal differences varied from 1.6 to 7.5 cm. The caliper technique consistently demonstrated the smallest mean absolute intraexaminer differences (0.68 to 1.39 cm) and the highest percentage of intraexaminer differences ≤ 1.0 cm (55% to 77.8%). The differences between pairs of examiners' measurements were consistently greater than the differences between individual examiner's measurements: Mean absolute differences varied from 1.36 to 3.60 cm, the percentage of differences ≤ 1.0 cm varied from 9.1% to 55.6%, and the maximal differences varied from 3.1 to 11.5 cm. Interexamniner differences were smallest for the tape “over-the-curve” measurements and the caliper measurements. These findings indicate that, to ensure maximal reliability, fundal height measurements should be obtained by the same clinician throughout pregnancy. Additionally, these findings indicate that calipers may be the most reliable method of obtaining fundal height measurements and that the accuracy of caliper measurements in identifying fetal growth disturbances and other pregnancy complications merits further investigation.

Computerized approach to monitoring aminoglycosides

Aminoglycosides are still used extensively in the treatment of nosocomial infections with Gram-negative bacteria. However, the treatment is associated with several adverse effects. Aminoglycosides monitoring is therefore essential to prevent toxic accumulations and to reach therapeutic concentrations. A computer program, PHARMONITOR, has been developed to optimize aminoglycosides monitoring, responding to the demands of most clinical daily situations. This program, based on a one-compartment open pharmacokinetics model, is developed for IBM PC-compatible computers, using D-Base III+. It can calculate t1/2, Vd, Cldrug, Cpmax, and the theoretical optimal dose and interval and also evaluates the creatinine clearance. The program has been conceived to allow maximal speed, flexibility, and reliability by the use of (e.g.) a linear least-squares analysis, the possible reference to previous protocols, the extensive use of keywords to classify and recall patients according to their pathologies, the development of messages recommending maximal dose or minimal dosing interval, and increasing the safety of the analysis. We consider the program a valuable tool for adjusting aminoglycoside dosage in individuals.