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PubMed1990

A statistical method for evaluating suicide clusters and implementing cluster surveillance.

The absence of any standard definition of suicide cluster events hinders understanding of the prevalence of the problem, hinders the development of appropriate public health responses to observed clusters, and ultimately hinders investigation of the mechanisms underlying contagious communication of suicidal behavior. The authors introduce a Poisson mixture model for assessing potential clusters of adolescent suicide, apply that model to the monthly incidence rates of adolescent suicide for one populous US county over the last 11 years, and generate 99% tolerance limits with 95% confidence for the number of suicides which may occur by chance within specific intervals of time in that county. The suicide incidence data showed a remarkable fit to a single Poisson distribution, suggesting it is not unreasonable to consider the cases as randomly-distributed and independent events. The authors conclude that there is no evidence that adolescent suicides occurred in clusters in the place and in the time frame under study, and recommend the Poisson mixture model for ascertaining clusters as well as implementing cluster surveillance.

Adolescent↗

PubMed1992

[Chiasma distribution in the lampbrush chromosomes of the chicken Gallus gallus domesticus: hot spots of recombination and their possible role in proper dysjunction of homologous chromosomes at the first meiotic division].

Chiasma distribution in the lambrush chromosomes of the chicken Gallus gallus domesticus was studied. The data of the authors show that the general pattern of chiasmata in the interstitional region of chromosomes corresponds to the Poisson distribution. However, in the telomeric and subtelomeric regions of all chicken macrochromosomes one can see chiasma as a rule. In the half of 140 microchromosomes from 24 different oocytes, there are also the telomeric chiasmata. On the basis of this observation, it may be predicted that there are hot spots of recombination near or into the telomeric GC-rich heterochromatic bands of chicken chromosomes. We suggest that these hot spots of recombination near the telomeres are a necessary facility for not only macrochromosomes but all microchromosomes as well to have at least one chiasma. The constant presence of at least one chiasma in a bivalent in needed for correct disjunction of homologous chromosomes at the first meiotic division.

Animals↗

PubMed1991

CANEST: a microcomputer program for estimating cancer in a cohort.

Certain diseases and symptoms carry an overrepresentation of cancer. To be able to measure the strength of such an association it is necessary to be able to predict cancer development in the group being observed. A computer program for computers running under the MS DOS operating system has been developed for this purpose. The program is written in the CLIPPER programming language. The estimates are based on incidence and prevalence data from the Swedish Cancer Registry for the years 1958 to 1986. The program also computes confidence intervals based on the Poisson distribution. The results can be printed out or exported to other programs for further analysis.

Cohort Studies↗

PubMed1991

An empirical Bayes formulation of cohort models in cancer epidemiology.

This paper concerns the incidence rates of malignant skin melanoma for several age-sex groups and time periods in three geographic regions, uses a method of cohort analysis and employs a two-stage random effects model. The first stage entails the assumption that the within-region variation in the frequency of disease incidence for a fixed age-sex-cohort group has a Poisson distribution with mean proportional to the population at risk. The second stage, after adjusting for age and sex, entails the assumption that the between-region geographic variation in the logarithm of the true incidence rate has a prior distribution with parameters estimated by the method of maximum likelihood. After adjusting for age effects, we estimate random geographic-specific cohort effects for each sex with use of an empirical Bayes method and compare the results with the usual multiplicative Poisson model that assumes fixed geographic-specific cohort effects for each sex. This comparison shows that the method presented here provides more stable estimates of geographic-specific cohort effects, and in addition the random effects model describes these data more adequately.

Adult↗

PubMed1992

Sister chromatid exchange frequency in breast cancer cases.

Sister chromatid exchange (SCE) were studied in 22 patients with breast cancer (i.e., four stage II; 12 stage III, six stage IV) and 10 normal healthy females as age-matched controls. The data obtained in these cases followed a Poisson distribution. An apparent increase in the average rate of SCE/cell was observed with the advancing stage of breast cancer.

Breast Neoplasms↗

PubMed1990

Paraplegic pressure sore frequency versus circulation measurements.

Paraplegic subjects (N = 34) were examined to determine the association of pressure sore history with respect to ankle pressure ratio and buttocks cutaneous plethysmographic harmonic persistence. No relationship was found between pressure sore history and ankle pressure ratio. No significant difference in ankle pressure ratios exists for those who have a pressure sore history as compared to those who have not experienced a pressure sore in 5 years. Those subjects with a diminished buttocks circulation harmonic persistence are more likely to have experienced one or more pressure sores than those subjects with normal circulation characteristics. A Poisson distribution analysis of multiple pressure sore occurrence suggests that repeated pressure sores are unlikely to arise as the result of chance.

Ankle↗

PubMed1992

The distribution of transposable elements within and between chromosomes in a population of Drosophila melanogaster. I. Element frequencies and distribution.

Data were collected on the distribution of nine families of transposable elements among second and third chromosomes isolated from a natural population of Drosophila melanogaster, by means of in situ hybridization of element probes to polytene chromosomes. It was found that the copy numbers per chromosome in the distal sections of the chromosome arms followed a Poisson distribution. Elements appeared to be distributed randomly along the distal sections of the chromosome arms. There was no evidence for linkage disequilibrium in the distal sections of the chromosomes, but some significant disequilibrium was detected in proximal regions. There were many significant correlations between different element families with respect to the identity of the sites that were occupied in the sample. There were also significant correlations between families with respect to sites at which elements achieved relatively high frequencies. Element frequencies per chromosome band were generally low in the distal sections, but were higher proximally. These results are discussed in the light of models of the population dynamics of transposable elements. It is concluded that they provide strong evidence for the operation of a force or forces opposing transpositional increase in copy number. The data suggest that the rate of transposition per element per generation is of the order of 10(-4), for the elements included in this study.

Animals↗

PubMed1992

Selectivity for temporal characteristics of sound and interaural time difference of auditory midbrain neurons in the grassfrog: a system theoretical approach.

The selectivity for temporal characteristics of sound and interaural time difference (ITD) was investigated in the torus semicircularis (TS) of the grassfrog. Stimuli were delivered by means of a closed sound system and consisted of binaurally presented Poisson distributed condensation clicks, and pseudo-random (RAN) or equidistant (EQU) click trains of which ITD was varied. With RAN and EQU trains, 86% of the TS units demonstrated a clear selectivity for ITD. Most commonly, these units had monotonically increasing ITD-rate functions. In general, units responding to Poisson clicks, responded also to RAN and EQU trains. One category of units which showed strong time-locking had comparable selectivities for ITD with both stimulus ensembles. A second category of units showed a combined selectivity for temporal structure and ITD. These units responded exclusively to EQU trains in a nonsynchronized way. From the responses obtained with the Poisson click ensemble so-called Poisson system kernels were determined, in analogy to the Wiener-Volterra functional expansion for nonlinear systems. The kernel analysis was performed up to second order. Contralateral (CL) first order kernels usually had positive or combinations of positive and negative regions, indicating that the contralateral ear exerted an excitatory or combined excitatory-inhibitory influence upon the neural response. Ipsilateral (IL), units were characterized by first order kernels which were not significantly different from zero, or kernels in which a single negative region was present. A large variety of CL second order kernels has been observed whereas rarely IL second order kernels were encountered. About 35% of the units possessed nonzero second order cross kernels, which indicates that CL and IL neural processes are interacting in a nonlinear way. Units demonstrating a pronounced selectivity for ITD, were generally characterized by positive CL combined with negative IL first order kernels. Findings suggested that, in the grassfrog, neural selectivity for ITD mainly is established by linear interaction of excitatory and inhibitory processes originating from the CL and IL ear, respectively. Units exhibiting strong time-locking to Poisson clicks and RAN and EQU trains had significantly shorter response latencies than moderately time-locking units. In the first category of units, a substantial higher number of nonzero first and second order kernels was observed. It was concluded that nonlinear response properties, as observed in TS units, most likely have to be ascribed to nonlinear characteristics of neural components located in the auditory nervous system peripheral to the torus semicircularis.

Acoustic Stimulation↗

PubMed1991

Temporal distribution of seizures in epilepsy.

A major problem in epileptology is why a seizure occurs at a particular moment in time. An initial step in solving this problem is a detailed analysis of the temporal distribution of seizures. Using methods and theories of stochastic processes, seizure patterns in a group of epileptic outpatients were examined for stationarity, randomness, dependency and periodicity in a prospective study. Sixteen of the 21 seizure diaries included in the study showed stationarity; 2 were non-stationary and 3 inconclusive. Eleven of the 16 stationary diaries were non-Poisson (P less than 0.005), indicating that in the majority of patients seizures did not occur randomly. The most frequently encountered phenomenon was seizure clustering. Clustering was considered when the diaries fulfilled all three criteria: (1) a positive R-test (P less than 0.001); (2) deviation from the fitted Poisson distribution towards clustering; and (3) the feature of an autoregressive process in the autocorrelogram plot. Dependency between seizure events was demonstrated in 8 of the 16 stationary diaries, computing first order transition probabilities. A detailed analysis of seizure occurrence is a major step towards a better understanding of the mechanisms underlying seizure precipitation. This is exemplified by our finding of a relation between seizure frequency and the menstrual cycle.

Adolescent↗

PubMed1989

Fragile sites and statistics.

A simple and reliable statistical method is proposed to distinguish real fragile sites from the casual accumulation of aberrations that can occur as a consequence of random distribution. It is based on a study of the expected random distribution, assumed to be equal to a Poisson distribution, with the expected value of events per band as the mean.

Chromosome Fragile Sites↗

PubMed1985

Direct analysis of radiation-induced chromosome fragments and rings in unstimulated human peripheral blood lymphocytes by means of the premature chromosome condensation technique.

Development of the procedure to stimulate peripheral blood lymphocytes has greatly facilitated the understanding of chromosome aberration formation and repair mechanisms in human cells. Yet, because radiation induces far more initial chromosome breaks than are observed as aberrations in metaphase, it has not been possible to examine the kinetics of primary chromosome breakage and rejoining with this procedure. An improved method to induce premature chromosome condensation in unstimulated lymphocytes has been used to study primary chromosome breakage, rejoining, and ring formation at various times after irradiation with up to 800 rad of X-rays. The dose-response relations for chromosome fragments analyzed immediately or 1, 2, or 24 h after exposure were found to be linear. Rapid rejoining of chromosome fragments, which takes place in the first 3 h after X-ray exposure, was not correlated with a simultaneous increase in the formation of rings. The yield of rings per cell scored 24 h after irradiation, however, increased significantly and fit a linear quadratic equation. Both chromosome fragment rejoining and ring formation were completed about 6 h after irradiation. The frequency distributions of rings among cells followed a Poisson distribution, whereas chromosome fragments were overdispersed.

Animals↗

PubMed1986

Induction of chromosome aberrations by 90Sr beta-particles in cultured human lymphocytes.

Human blood was irradiated with beta-particles from an external source of 90Sr. The source was a rolled piece of silver foil, active dimensions: 100 X 12.5 mm, incorporating 3.7 X 10(8) Bq (10 mCi) of 90Sr/90Y. After culturing for 48 h, the dicentric yield in the lymphocytes at the first metaphase was measured as a function of the dose in the blood. The aberration yield fitted the linear-quadratic function well, which is consistent with the single-track and two-track model for aberration formation at low LET radiation. The curve for 90Sr beta-rays was compared with a curve for 60Co gamma-rays. The main difference between the coefficients was in the alpha values. With respect to 60Co gamma-rays, the RBE calculated from the dose-effect relationships for dicentric production was 2.8 at the dose of 0.14 Gy; it decreased with increasing doses. The distribution of dicentrics was consistent with the Poisson distribution but showed a tendency to over-dispersion in the region of higher doses. A reason for these discrepancies is discussed.

Beta Particles↗

PubMed1979

Change of statistical parameters of transmitter release during various kinetic tests in unparalysed voltage-clamped rat diaphragm.

1. The statistical nature of transmitter release was studied in unparalysed cut rat diaphragm using a voltage clamp technique at room temperature (23 degrees C). 2. While the binomial distribution described observed amplitude histograms well, the Poisson distribution was clearly inadequate. Values of m ranged from 40 to 45, while values for n varied from 45 to 53 and p was between 0.82 and 0.90. 3. Estimation of the probability of release from the transient decay of e.p.c.s in short tetanic trains (11 pulses, 150 Hz) gave values of p from 0.031 to 0.054, which are more than one order of magnitude lower than statistical estimates. 4. As a result of the short tetanic stimulation (5 pulses, 20, 50 and 100 Hz) there is an initial transient facilitation which afterwards becomes masked by depression. Statistical analysis suggests that the changes in the average numbers of quanta released (m) could be attributed to the change in the immediately available store (n). 5. During long tetanic stimulation (4000 pulses, 10-100 Hz) statistical analysis suggests that the decrease in the average number of quanta released (m) could be attributed almost entirely to the decrease in the immediately available store (n). The probability of release (p) decreased only slightly. The extent of the post-tetanic potentiation indicates that it cannot be explained on the grounds of increased probability of release (p) only. There should be an increase as well in the immediately available store (n). 6. It is suggested that while depression is most likely caused by the depletion of the immediately available store due to insufficient replenishment, the facilitation is probably caused by the increase in the capacity of the immediately available store to contain transmitter.

Animals↗

PubMed1991

Modeling the overdispersion of parasite loads.

Many epidemic models lead to an approximately Poisson distribution of parasites among hosts. This is at variance with observation, where heavy overdispersion is the rule. A simple model is proposed that, while treating individuals alike, nonetheless gives rise to highly variable parasite loads.

Animals↗

PubMed1992

An inferential procedure for the Poisson intervention parameter.

To describe a health chance mechanism whose incidence rate is altered in the middle of a data collection period due to preventive treatments taken by health service agencies, a model called Intervened Poisson Distribution (cf. Shanmugam, 1985, Biometrics 41, 1025-1029) was introduced. In this article, we derive a statistic to test the hypothesis concerning the effectiveness of such preventive treatments, and compare its power with another suggested test (cf. Streit, 1987, Biometrics 43, 999-1000).

Health Services↗

PubMed1992

Repeatability and reproducibility of egg counts of Schistosoma haematobium in urine.

Egg output in urinary schistosomiasis can be influenced by several factors, such as time of collection of urine, day to day variations, seasonal variations and environmental conditions. Additionally there are unavoidable random errors inherent in every test procedure. To assess the precision of Schistosoma haematobium egg counts, carried out after paper filtration, two measures are determined, namely repeatability (r) and reproducibility (R). 10 aliquots of 10 urine samples are examined by 5 microscopists and values for r and R are calculated following the methods of the British Standards Institution (BSI, 1987). Results show unexpectedly high values for the 2 measures in relation to the overall means in all 10 urine samples, and analyses of variance confirm that there are large variations between aliquots of equal volume drawn from the same urine specimen, highly significant in all 10 urine specimens; variations between microscopists are considerably lower and only highly significant in one urine specimen. The Poisson heterogeneity test for all urine samples refutes in 9 out of 10 cases the hypothesis that aliquots of equal volume taken from a well mixed urine sample could reasonably be drawn from a Poisson distribution. It is therefore concluded that egg counts are very valuable for community studies and should always be performed, but results must be interpreted with more caution than before and great care taken in analyzing counts (especially in longitudinal studies), in classifying them and in relating them to morbidity.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Murphy's law of limiting dilution cloning.

Conventional practice and literature on limiting dilution cloning address the irrelevant problem of selection of a single progenitor from a uniform population, and provide optimistic estimates of monoclonality for interesting cultures and subcultures. Any cell line established via these estimates is suspect and may be polyclonally metastable. Cultures derived by limiting dilution of the progenitors of independently Poisson distributed populations obey a counterintuitive relation with the characteristics of a Murphy's law: the probability that an interesting culture is monotypic or monoclonal is less than that of a random non-sterile culture, decreases for increasingly rare interesting cultures, and is bounded below by the probability of sterility. A priori and empiric a posteriori estimates of the probability that interesting subcultures are monotypic or monoclonal are derived consistent with this principle.

Cells, Cultured↗

PubMed1989

Logistic and Poisson models for infection by multicomponent plant viruses.

A model for the relationship between virus concentration and infectivity of multicomponent plant viruses is based on a combination of logistic and Poisson equations. Two separate equations are derived from the Poisson distribution assuming, (i) that infections occur only when a set of components containing the complete multicomponent genome is established at an infection site, but that any excess of components present does not reduce the probability of infection (no interference postulate); and (ii) that infection can occur only if a set of components containing the full genome reaches an infection site before it can be preempted by an incomplete set (competitive interference postulate). Postulate (i) affects the form of a dilution series without affecting N, the maximum possible number of infections (lesions), and postulate (ii) changes the value of N but not the form of the dilution series. There is a close correlation between the logit slope of a logistic dilution series and the form of the corresponding multiple Poisson dilution series for viruses with 2, 3 or 4 components. Calibrated by Poisson equations, the logit slope may thus suggest whether or not the virus components have invaded independently and infected similar infection sites. The methods of fitting the combined logistic-Poisson model are demonstrated by applying it to data for cowpea chlorotic mottle virus.

Models, Theoretical↗