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PubMed1995

The effect of mating structure and progeny distribution on heterozygosity versus the number of alleles as measures of variation.

Heterozygosity and the number of alleles are both measures of the genetic variation of a population. They are qualitatively similar if the distribution of the number of progeny is Poisson, but not necessarily for other distributions. In particular, selfing decreases heterozygosity and also decreases the number of alleles when the progeny distribution is Poisson, but decreases heterozygosity and increases the number of alleles when there are two progeny per individual. This is because heterozygosity is closely related to the breeding structure (inbreeding) of the population, whereas the number of alleles is more influenced by the variation of the sampling process. Branching processes are employed to model the dynamics of mutant alleles, with drift and subsequent mutation contributing to extinction. In populations of four individuals, double first cousin mating has greater heterozygosity, but fewer alleles, than half-sib mating.

Alleles↗
PubMed2006

A simulation model for the study of the within-animal infection dynamics of E. coli O157.

Escherichia coli 0157 can cause serious illness, even death, in humans. There is some consensus that the main reservoirs of this harmful bacterium are the rumens and intestines of cattle. Hence, a stochastic model of the bovine gut was developed to investigate the in vivo population dynamics of E. coli O157. Because bacterial numbers can reach minimal levels, a stochastic system was considered, with a birth-death process being used to represent bacterial growth and decay dynamics throughout the gastrointestinal tract. Reinfection through ingestion of bacteria present in the environment was allowed to occur and the required clustered distribution of inter-event times was implemented through the use of a random hazard doubly stochastic Poisson process. Due to the inclusion of multiple compartments, a feedback mechanism and an interest in the non-equilibrium dynamics of the process, it was not possible to obtain an analytical representation of the process and therefore, a simulation study was used to obtain results. The within-animal model can be used to explore the efficacy of control measures which act at an individual animal level.

Animals↗
PubMed2001

Changes and trends in attack distributions and progression of dental caries in three age cohorts in Finland.

BACKGROUND: Several factors are claimed to have contributed to the decline in dental caries that has occurred over recent decades in many industrialised countries. METHODS: A retrospective cohort design follow-up study of trends in dental caries in three age cohorts born a decade apart is reported from Finland. Subjects born in the 1960s, 1970s and 1980s (n = 1275) were monitored annually through their patient records, and changes in the distribution, extent and rate of caries attack (caries leading to restorations or extractions) were analysed. Logistic and Poisson regression techniques were employed to detect trends and Kaplan-Meier survival methods were used for the rate analyses. RESULTS: A marked decrease in caries was observed and the proportion of disease-free subjects increased gradually towards the younger cohorts. Logistic regression analysis showed clear trends, in that the odds ratios (ORs) for the cohort effect were 8.93 [95% confidence interval (CI) = 7.95-10.04] and 4.32 (95% CI = 3.92-4.75) in the 1960 and 1970 cohorts, respectively, relative to the 1980 cohort, and that for the age (year) effect was 1.44 (95% CI = 1.42-1.46). Similar types of cohort and age effects (p < 0.0001) were found in the disease progression analyses. The rate analyses showed statistically highly significant differences between the three cohorts (p < 0.001) for both sexes. The caries decline was a lasting one, in spite of the delay in restorations observed in the youngest cohort. DISCUSSION: The results indicate a vast and continuous trend in the incidence of dental caries leading to restorations and extractions. Significant changes in the rate and extent of disease progression have taken place, which will inevitably affect the future public health agenda.

Adolescent↗
PubMed1974

Effect of structure on function in model nerve nets.

A theoretical analysis has been made on the effect of the pattern of interneuronal connectivity in model nerve nets on the activity of these nets. Two types of nets have been investigated: one in which the likelihood of a connection between a given neuron and any other element in the net is given by a Poisson probability distribution, and a second type in which the pattern of interconnection follows a Gaussian distribution. An analytical treatment is presented of the equations for noiseless nets in these two conditions. The principal result is that nets with Poisson connectivity law are activated by extraneous firing of a single neuron and continue in spontaneous activity indefinitely. On the other hand, similar nets in which the connections are, however, distributed according to a normal connectivity law, exhibit a definite threshold and produce spontaneous activity only subsequent to extraneous activation of a substantial fraction of the population. Moreover, spontaneous activity in Gaussian nets, but not in Poisson nets, becomes extinguished if the number of active neurons falls below the critical threshold. Some neuroanatomical implications are discussed which suggest that the pyramidal system of the cerebral cortex and other neuronal systems histologically characterized by large numbers of synapses per neuron may incorporate a Gaussian connectivity law, whereas a Poisson law may be characteristic of these cortical layers and nuclei primarily containing granule cells.

Electric Conductivity↗
PubMed1996

Mixture models for eye-tracking data: a case study.

Heterogeneity in biomedical data is often a source of great scientific interest and mixture models provide a general framework for modelling the various types that arise in practice. Finite mixture models model discrete subgroups within populations while continuous mixture models inflate the variance to account for over-dispersed data. A potential problem with the application of finite mixture models in practice is that these models may drastically overestimate the number of component densities when there is a lack of model fit. This can have severe consequences, leading the data analyst to attach substantive interpretations to spurious subgroups. For this reason, we propose using the continuous mixture model as an alternative when fitting finite mixture models with an arbitrary number of components. In the context of an example examining a specific oculomotor component of eye-tracking dysfunction in schizophrenia, we demonstrate why the continuous mixture model provides a viable alternative to the finite mixture model for small sample sizes. We present methods for fitting and comparing both models using the parametric bootstrap and EM algorithm, and show that the distinction between the models decreases as the number of component densities in the finite mixture model increases.

Algorithms↗
PubMed1989

Analysis of epidemiologic case-base studies for binary data.

Recent developments in statistical methods for epidemiology have revived the application of sampling techniques in the design and analysis of cohort studies. The 'case-base' design involves sampling of both the cases and the cohort base of the study. This paper reviews some data-analytic imperfections of the approaches to risk ratio estimation, and modifies and advances a consistent likelihood-based procedure-analogous to Miettinen and Nurminen's proposal for a full cohort design--for interval estimation (and also point estimation and significance testing) in the context of binary case-base data. First, the procedure avoids the use of Taylor-series approximations to derive variance estimators for non-linear functions of parameters. Second, the asymptotic condition effects a simple computational expression for the chi-square function of risk ratios that is universally applicable to small samples. The statistical modelling underlying the method allows inferences about risk ratios without the assumption of rare disease either for the general population or for a particular base. The paper also extends the analysis to encompass stratified data. Finally, a numerical evaluation evinced the accurate small-sample properties of the proposed method.

Binomial Distribution↗
PubMed1995

Effect of roadway geometrics and environmental factors on rural freeway accident frequencies.

This paper explores the frequency of occurrence of highway accidents on the basis of a multivariate analysis of roadway geometrics (e.g. horizontal and vertical alignments), weather, and other seasonal effects. Based on accident data collected in the field, a negative binomial model of overall accident frequencies is estimated along with models of the frequency of specific accident types. Interactions between weather and geometric variables are proposed as part of the model specifications. The results of the analysis uncover important determinants of accident frequency. By studying the relationship between weather and geometric elements, this paper offers insight into potential measures to counter the adverse effects of weather on highway sections with challenging geometrics.

Accidents, Traffic↗
PubMed2003

Assessment of a national health interview survey-based method of measuring community socioeconomic status.

PURPOSE: Increasingly researchers are interested in assessing the role of community socioeconomic status (SES) in poor health outcomes, above and beyond the influence of low individual SES. However, the feasibility of conducting these multi-level studies is often limited by restrictions on release of confidential identifiers for linkage to census data, resources for the linkage, and the availability of data sources with individual SES measures. This study assessed a new method of measuring community socioeconomic status (SES) that can be used with the publicly available National Health Interview Survey (NHIS) and preserves confidentiality and can be used with individual SES measures from the NHIS. METHODS: The associations between community SES and mortality from all causes and breast cancer in women were assessed in two samples: 1) deaths in 1987-1993 NHIS respondents linked to community SES measures developed with the new method; and 2) deaths in 1991 from the National Multiple Cause of Death Files linked to 1990 county-level census SES measures. The magnitude of crude mortality rates, direction of trend, and age-adjusted relative risk of mortality for low vs. high SES were compared in the two samples. RESULTS: Crude all-cause mortality and breast cancer mortality rates were similar in both samples in terms of magnitude and direction of trend. In both samples, as SES decreased, rates of all-cause mortality increased, whereas breast cancer mortality rates tended to decrease. Age-adjusted relative risks of mortality from all causes and breast cancer for low vs. high SES were similar in the two samples. CONCLUSIONS: Similarity of associations between community SES and mortality from all causes and breast cancer in the two samples provides support for the validity of a new NHIS-based method of measuring community SES. Since the NHIS is a large, nationally representative survey with high response rates and low loss to mortality follow-up, this method represents an important resource for multi-level studies.

Aged↗
PubMed1991

The role of positive and negative interspecific associations in the organization of communities of intestinal helminths of bats.

Twelve populations of bats were examined to determine the extent of interspecific associations in determining the species richness of intestinal helminth infracommunities. The pool of helminth species which was available to individual bats ranged from 2 to 21. The 'summed binomial' distribution was determined to underlie the host frequency distribution of the number of helminth species per host. Overall covariation in occurrences of species in replicated communities can be detected by testing for the equality of the observed variance of the host frequency distribution to the variance expected when species are allocated to hosts at random. Where statistically significant the covariance was indicative of a majority of positive rather than negative interspecific associations. As the mean number of species per host in a host population increases not only does the number of positive associations increase but so does the proportion of species pairs which exhibit positive associations. Although there is an increase in the proportion of species pairs which exhibit positive associations as the number of species increases, the magnitude of the associations (as indicated by the mean positive or the mean negative pairwise covariances) does not. Therefore, we concluded that positive interactions are more common than negative interactions in determining the species richness of helminth infracommunities of bats. Further, positive associations become even more important as the community becomes more complex. However, the increased importance is derived from the number rather than the strength of the associations.

Animals↗
PubMed1997

Occupation and cervical cancer: an opportunity for prevention.

Cervical cancer remains an important health problem for women. Few published studies have examined cervical cancer with respect to a woman's occupation. This study examines the association of cervical cancer mortality and occupation in a large national database. The purpose of the study is to recommend which occupations may most require health promotion activities. Mortality data from the National Occupational Mortality Surveillance System were used to calculate the proportion of deaths from cervical cancer according to occupation. This study is based on standardized death certificate data for almost 2 million deaths among women in 27 states, covering the period 1985-1990. Our results are consistent with those in previous studies, with service and apparel manufacturing workers showing elevated risk. Data presented show a difference in cervical cancer mortality by occupational group. Identification of these occupations suggests which women could be targeted for preventive services. Women in occupations with low socioeconomic status are less likely to have access to health promotion programs. Resources should be directed to these women.

Adult↗
PubMed2006

Recent national trends in sudden, unexpected infant deaths: more evidence supporting a change in classification or reporting.

The recent US decline in sudden infant death syndrome (SIDS) rates may be explained by a shift in how these deaths are classified or reported. To examine this hypothesis, the authors compared cause-specific mortality rates for SIDS, other sudden, unexpected infant deaths, and cause unknown/unspecified, and they evaluated trends in the age and month of death for these causes using 1989-2001 US linked birth/death certificate data. Reported deaths in state and national data were compared to assess underreporting or overreporting. SIDS rates declined significantly from 1989-1991 to 1995-1998, while deaths reported as cause unknown/unspecified and other sudden, unexpected infant deaths, such as accidental suffocation and strangulation in bed (ASSB), remained stable. From 1999-2001, the decline in SIDS rates was offset by increasing rates of cause unknown/unspecified and ASSB. Changes in the cause-specific age at death and month of death distributions suggest that cases once reported as SIDS are now being reported as ASSB and cause unknown/unspecified. Most of the decline in SIDS rates since 1999 is likely due to increased reporting of cause unknown/unspecified and ASSB. Standardizing data collection at death scenes and improving the reporting of cause of death on death certificates should improve national vital records data and enhance prevention efforts.

Cause of Death↗
PubMed1997

Environmental equity in Superfund. Demographics of the discovery and prioritization of abandoned toxic sites.

This article presents findings of the first national tract-level analysis of the distribution of residential characteristics, including the percentage of selected minorities and socioeconomically disadvantaged groups, in relation to uncontrolled toxic waste sites (i.e., CERCLIS and NPL sites). In contrast to prevailing notions, the authors find that uncontrolled toxic waste sites are not disproportionately located in minority neighborhoods or neighborhoods of socioeconomically disadvantaged residents. However, multivariate analyses of site distribution and a hazard regression analysis of the site prioritization process suggest that communities with a higher percentage of Black residents are less likely to receive NPL (National Priorities List) designation, delaying potential remediation. Biases in the prioritization process are, however, substantively small.

Censuses↗
PubMed1997

Risk analysis in relation to the importation and exportation of animal products.

The design of a quantitative risk analysis model has to be dictated by the questions it seeks to answer. The model should also be as objective as the available data will allow. Animal and animal product import risks usually have three characteristics which make the design of a good quantitative risk analysis model quite difficult, namely:--the probabilities of the steps leading to the undesired outcome are frequently inter-related--the probability of the undesired outcome itself is in many cases very small, making direct simulation impractical--important variables within the model often cannot be quantified through analysis of data, thus these variables must be modelled with probability distributions to reflect the degree of uncertainty, usually determined by expert opinion. This paper provides a tutorial on some modelling techniques which are essential to the risk assessment of animal and animal product imports and which help overcome these problems. A number of probability distributions, their uses and inter-relationships, are examined. The application of these distributions, coupled with some general modelling techniques, is then demonstrated to produce rigorous and transparent animal import risk analyses.

Animal Diseases↗
PubMed2001

Iterated birth and death process as a model of radiation cell survival.

The iterated birth and death process is defined as an n-fold iteration of a stochastic process consisting of the combination of instantaneous random killing of individuals in a certain population with a given survival probability s with a Markov birth and death process describing subsequent population dynamics. A long standing problem of computing the distribution of the number of clonogenic tumor cells surviving a fractionated radiation schedule consisting of n equal doses separated by equal time intervals tau is solved within the framework of iterated birth and death processes. For any initial tumor size i, an explicit formula for the distribution of the number M of surviving clonogens at moment tau after the end of treatment is found. It is shown that if i-->infinity and s-->0 so that is(n) tends to a finite positive limit, the distribution of random variable M converges to a probability distribution, and a formula for the latter is obtained. This result generalizes the classical theorem about the Poisson limit of a sequence of binomial distributions. The exact and limiting distributions are also found for the number of surviving clonogens immediately after the nth exposure. In this case, the limiting distribution turns out to be a Poisson distribution.

Cell Survival↗
PubMed2004

Random networks with tunable degree distribution and clustering.

We present an algorithm for generating random networks with arbitrary degree distribution and clustering (frequency of triadic closure). We use this algorithm to generate networks with exponential, power law, and Poisson degree distributions with variable levels of clustering. Such networks may be used as models of social networks and as a testable null hypothesis about network structure. Finally, we explore the effects of clustering on the point of the phase transition where a giant component forms in a random network, and on the size of the giant component. Some analysis of these effects is presented.

Journal Article↗
PubMed2003

Incidence of malignant disease by morphological type, in young persons aged 12-24 years in England, 1979-1997.

Cancer incidence data are generally presented in terms of primary site, but this method is inappropriate for cancers in young persons. We have used a morphology-based classification system to produce national incidence rates for cancers in persons aged 12-24 years by detailed diagnostic sub-type. The overall incidence rates for malignant disease in young persons aged 12-14, 15-19 and 20-24 years were 10.1, 14.4 and 22.6 per 100000 population, respectively. The three most frequent cancer types in 12-14-year-olds were leukaemias, lymphomas and central nervous system (CNS) tumours. In 15-19-year-olds, lymphomas were most frequent and leukaemias second with carcinomas third. In 20-24-year-olds, lymphomas were again most frequent, but carcinomas and germ cell tumours were second and third. There was also variation with age in the ratios of rates in males and females. These changing incidence patterns have aetiological implications and provide clues for future hypothesis-based research.

Adolescent↗