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At least 343 records · Page 19Linked to original sources
PubMed1987

A high-conductance calcium-dependent chloride channel in Ascaris suum muscle.

The properties of a high-conductance Ca-dependent Cl channel in the body muscle membrane of the nematode parasite Ascaris suum were examined with the patch-clamp technique. In Ascaris Ringer solution the current-voltage relationships of the channel appeared linear during hyperpolarization with a slope conductance of 200 pS (in symmetrical 175 mM-Cl): during depolarization the slope conductance reduced. Cation and anion replacement experiments showed this channel to be permeable to Cl ions. In artificial perienteric fluid (in symmetrical 78 mM-Cl) the channel conductance was found to be 114 pS. Subconductance states were seen when the patch was depolarized. The channel conductance showed saturation with increasing Cl concentrations. The channel was voltage sensitive: the probability of the channel being open and the mean open time of the channel reduced when the patch was depolarized. Mean channel open times between 2.4 and 215 ms were observed. Frequency histograms of the distributions of open and closed times were fitted with two and three exponentials respectively; this suggests complex channel kinetics with multiple open and closed states. Ca applied to the intracellular surface of the patch increased in a dose-dependent manner the number and probability of the channels being open. These channels were seen in about 20% of all patches and occasionally up to 15 channels were seen in a single patch. The distributions of the probabilities of seeing N channels open in multichannel patch records were not not always well fitted by the binomial distribution: it is suggested that adjacent channels could have different probabilities of being open. The possible functions of this channel are discussed.

Animals↗
PubMed2004

Aggregation of helminths: the role of feeding behavior of fish hosts.

Individual Arctic charr (Salvelinus alpinus) from Fjellfrøsvatn, northern Norway, could be categorized by their stomach contents as zooplanktivores or benthivores. Feeding specialization among these fish was evident from negative correlations between helminths transmitted by pelagic copepods (Diphyllobothrium dendriticum and D. ditremum) and those transmitted by the benthic amphipod Gammarus lacustris (Cystidicola farionis and Cyathocephalus truncatus). Occurrences of parasite species acquired from the same types of invertebrate were positively correlated in the fish. Strong relationships among habitat use, diet, and helminth infections among the Arctic charr indicated persistent foraging patterns involving long-term habitat use and feeding specialization. The distribution of all parasite species was highly aggregated in the fish samples, measured by the exponent k of the fitted negative binomial distributions (range: 0.5-7.5) and the variance-to-mean ratios (s2/mean, range: 5-85). Charr specializing on either copepods or Gammarus predominantly contributed to high-intensity class intervals within the overall frequency distributions of the corresponding parasite species. Such fish had low infection intensities of helminths transmitted by other prey organisms. The detailed analyses of the parasite frequency distributions for fish with different habitat or feeding preferences evidently show how heterogeneity in trophic behavior contributes strongly to the commonly observed aggregation of helminths among hosts under natural conditions.

Amphipoda↗
PubMed1987

Gastrointestinal nematode parasites in Saskatchewan cattle: egg count distributions in beef animals.

Fecal samples were collected systematically in the spring from cows and yearlings entering, and in the fall from calves leaving, five community pastures in central Saskatchewan. Fecal samples were also collected systematically in the spring from cows entering, and in the fall from calves leaving, an experimental rotational grazing system. Samples were collected from 1398 animals and were examined by a quantitative fecal flotation technique with a sensitivity of ten eggs per gram. Distributions of nematode egg counts for 11 of the 12 cattle populations sampled were well described by a series of negative binomial distributions. A common value of k (a measure of aggregation) could be fitted to the counts from animals of all ages in four of the five community pastures, as well as to the counts from the cows from the fifth community pasture and from the rotational grazing system. A second value of k could be fitted to the counts from the calves from these two pasture. In addition, in three of the community pastures animals in the different age groups had the same mean count.

Animals↗
PubMed1987

Terminal distributions along a 'knight's line' for a stochastic epidemic.

The model under consideration relates to the spread of a disease in a finite population where removal of infectives is allowed to occur. It is equivalent to a restricted random walk over a discrete grid of points interior to a trapezium. We consider an artificial barrier on the grid at which the epidemic ceases. The barrier is a semi-diagonal resembling the path of the knight in chess and allows some simplification of the analysis. In particular, we investigate the probability distribution along this barrier when time is infinite. Recursive relations are obtained for these terminal distributions and are used to develop an exact numerical method for their computation. In certain cases, a slight modification of the binomial distribution provides a good approximation to the actual distribution. A direct recursive method for calculating moments of the distribution is given and we show how the whole distribution may be recovered from these moments.

Disease Outbreaks↗
PubMed2005

Statistical methods for the analysis of two-arm non-inferiority trials with binary outcomes.

The aim of this contribution is to give an overview of approaches to testing for non-inferiority of one out of two binomial distributions as compared to the other in settings involving independent samples (the paired samples case is not considered here but the major conclusions and recommendations can be shown to hold for both sampling schemes). In principle, there is an infinite number of different ways of defining (one-sided) equivalence in any multiparameter setting. In the binomial two-sample problem, the following three choices of a measure of dissimilarity between the underlying distributions are of major importance for real applications: the odds ratio (OR), the relative risk (RR), and the difference (DEL) of both binomial parameters. It is shown that for all three possibilities of formulating the hypotheses of a non-inferiority problem concerning two binomial proportions, reasonable testing procedures providing exact control over the type-I error risk are available. As a particularly useful and versatile way of handling mathematically nonnatural parametrizations like RR and DELTA, the approach through Bayesian posterior probabilities of hypotheses with respect to some non-informative reference prior has much to recommend it. In order to ensure that the corresponding testing procedure be valid in the classical, i.e. frequentist sense, it suffices to use straightforward computational techniques yielding suitably corrected nominal significance levels. In view of the availability of testing procedures with satisfactory properties for all parametrizations of main practical interest, the discussion of the pros and cons of these methods has to focus on the question of which of the underlying measures of dissimilarity should be preferred on grounds of logic and intuition. It is argued that the OR clearly merits to be given preference also with regard to this latter kind of criteria since the non-inferiority hypotheses defined in terms of the other parametric functions are bounded by lines which cross the boundaries of the parameter space. From this fact, we conclude that the exact Fisher type test for one-sided equivalence provides the most reasonable approach to the confirmatory analysis of non-inferiority trials involving two independent samples of binary data. The marked conservatism of the nonrandomized version of this test can largely be removed by using a suitably increased nominal significance level (depending, in addition to the target level, on the sample sizes and the equivalence margin), or by replacing it with a Bayesian test for non-inferiority with respect to the odds ratio.

Biopharmaceutics↗
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↗
PubMed1988

The relation between the number of parasites/host and host age: population dynamic causes and maximum likelihood estimation.

We examined dynamical factors that shape the distribution of the number of parasites/host in constant or temporally varying environments, and with or without host-age dependent variation in host susceptibility and parasite mortality. We predict properties of the parasite distribution in the absence of density-dependent factors such as density-dependent mortality of recruitment and parasite-induced host mortality. These properties provide a criterion for the detection of density dependence in temporally variable systems with host-age dependent interactions. We have then introduced methods to estimate and statistically evaluate the effects of host age or size on the distribution of parasites/host. The methods are based on a maximum likelihood protocol for linear and non-linear regression when data are negatively binomially distributed. We have illustrated the use of the theoretical results and statistical methods by re-analysing the data of Halvorsen & Andersen (1984) on cestode infections in Norwegian arctic charr and by analysing new data on nematode infections in Caribbean Anolis lizards.

Age Factors↗
PubMed1992

Quantal parameters of "minimal" excitatory postsynaptic potentials in guinea pig hippocampal slices: binomial approach.

Binomial distributions of amplitudes of excitatory postsynaptic potentials (EPSPs) mixed with Gaussian noise were simulated. The objective of Monte Carlo simulations was, firstly, to study influences of sampling size (N) and noise standard deviation (Sn) on estimates of mean quantal content (m), quantal size (v) and binomial parameters (n and p) by four methods of quantal analysis (histogram, variance, failures and combined method) based on the binomial model and, secondly, to modify these methods on the basis of comparison of estimated with simulated parameters. Reliable estimates (within +/- 10% of the simulated values) were obtained for large sample sizes (N = 500-1000) with Sn less than or equal to v by the histogram (deconvolution) method and with Sn less than or equal to 2v by the other three methods. Similar results were obtained by averages from about 10 simulations if smaller samples were used (N = 50-200). In electrophysiological experiments on slices, "minimal" EPSPs were recorded from CA1 pyramidal cells after low-intensity stimuli to stratum radiatum or stratum oriens. Amplitudes of minimal EPSPs fluctuated in a manner predicted by the quantum hypothesis. Amplitude distributions of EPSPs in the non-facilitated state were adequately described either by binomial statistics with an average p equal to about 0.4 (a range of 0.3-0.7) and an average n of about 3 (range 2-6) or by Poisson statistics with m of about 1. The quantal analysis suggests that typical values of m and v for a single activated fibre in stratum radiatum might be about 0.5-1 and 300-400 microV, respectively, with low p (0.1-0.3) and n (2-4). However, the estimates of binomial parameters should be considered as coarse approximations in view of the simulation results and a possible nonuniformity of parameter p. The comparison of results of various methods based on the binomial model, in both simulation and physiological experiments, indicates the reliability of estimates of basic quantal parameters (m and v) under realistic conditions of physiological experiments. The methods are considered to be sufficiently sensitive to make use of them for studies on mechanisms of long-term synaptic plasticity.

Animals↗
PubMed1985

Heterotrophic bacteria in water distribution systems. II. Sampling design for monitoring.

In this paper, which is a continuation of the work presented in Part I in this issue, previous information on the spatial and temporal variability of bacteriological data from a water distribution system is used to develop a sampling design for use in future water quality monitoring. The water distribution system is considered to be composed of several zones where the variation of bacterial counts in each zone is modelled by the negative binomial distribution. Under the assumption that the objective of monitoring is to determine whether or not the mean bacterial density of the water exceeds a specific standard, a criterion is given which determines the optimal number of sampling stations allocated to each zone. These stations are determined by assuming that either the risk of sampling (i.e. making the wrong decision) is prespecified or that the total number of stations to be sampled is predetermined. Sequential sampling to evaluate the compliance of the water with the standard is also discussed.

Bacteria↗
PubMed1980

Mathematical study of the contamination phenomenon in the culture of tubercle bacilli.

This paper demonstrates that when 'spot' specimens of sputum are cultured for tubercle bacilli on Löwenstein-Jensen medium, bacterial contamination occurs at random and follows a Poisson distribution. Over a period of 14 years, contamination rates with overnight 'collection' specimens (4.5 to 10.2%) were substantially higher than with spot specimens (1.3 to 4.0%), but the correlation between the two was very high (r = 0.98). In contrast to spot specimens, the incidence of contamination with collection specimens was significantly higher for females than for males, and significant variation was also found between patients of the same sex. In consequence, the Poisson distribution, which assumes a constant contamination rate for all patients, did not provide a satisfactory fit to the data on collection specimens. However, a negative binomial distribution, which incorporates individual variability in contamination rate, provided a good fit. The reasons for individual variability are discussed.

Bacteriological Techniques↗
PubMed1976

Analysis of intercellular distributions of chromatid aberrations.

59 intercellular distributions of chemically induced and spontaneous chromatid aberrations were analyzed for goodness of fit in respect of the Poisson (PD), the geometrical (GD), and the negative binomial distributions (NBD). The data are excellently described by the NBD. This distribution can be deduced from a model out of the queueing theory and is based on the hypothesis of restitution. Estimators are obtained for the ratio of restitution and induction processes involved in the origin of chromosomal aberrations.

Alkylating Agents↗
PubMed1984

Non-random distribution of udder infections among cows. Evaluation of some contributing factors.

Bovine udder infections were recorded in three herds throughout a lactation period with bacteriological examinations at 3 week intervals. For analysis, each cow was taken into account at the time when the greatest number of its quarters were infected. Distributions of cows having 0, 1, 2, 3, or 4 infected quarters, either by major or minor pathogens, differed significantly (P less than 0.01) from the random binomial distribution: the number of cows with 3 or 4 infected quarters was much higher than expected. As the two types of pathogens behave similarly, they were not distinguished and data were pooled. Distributions of infections among cows within each of the three herds were not random. Cross-infections (from a quarter to another of the same cow) could have given rise to less than one-third of the new infections. A greater proportion of cows became multi-infected with increasing age. This process was delayed in the herd with the lowest new infection rate. In the three most highly represented lactation ages (1, 2, and 4), differences between actual and theoretical distributions were all significant (P less than 0.01). Results suggested that together with the major contribution of concentration of infections in the older cows and the less important contribution of cross-infections, individual susceptibility played a part in the non-random distribution. Nevertheless the resistance of the less susceptible cows seemed to have been easily overcome when measures for control of new infections were not used.

Age Factors↗
PubMed1999

Sister chromatid exchange data fit with a mixture of Poisson distributions.

Bowman et al. [K.O. Bowman, Wesley Eddings, Marvin A. Kastenbaum, L. R. Shenton. Sister chromatid exchange data and Gram-Charlier series, Mutat. Res., 403 (1998) 159-169.] have shown that a Gram-Charlier modification of a negative binomial distribution gives a reasonable fit to counts of sister chromatid exchange (SCE) data originally presented by Bender et al. [M.A. Bender, R.J. Preston, R. C. Leonard, B.E. Pyatt, P.C. Gooch, On the distribution of spontaneous SCE in human peripheral blood lymphocytes, Mutat. Res., 281 (1992) 227-232. ]. Here we show that a mixture of a generalized Poisson distributions also fits the data. Advantages of the generalized Poisson mixture include a simplified model involving only four parameters which fits the data more closely according to the chi-squared goodness-of-fit criterion.

Female↗
PubMed1979

Neurotransmitter release statistics: moment estimates for inhomogeneous Bernoulli trials.

Evoked release of quanta of neurotransmitter is generally treated as a set of homogeneous, stationary Bernoulli trials, hence governed by the binomial distribution. Relaxing the assumptions of uniformity and stationarity leads to a more realistic physiological model of transmitter release but also introduces systematic biases in the moment estimates of the binomial parameters. We derive probability generating functions for quantal release and expressions for the moment estimates of n and p for a generalized model that incorporates temporal variation and nonuniformity in individual release probabilities and in numbers of release sites.

Action Potentials↗
PubMed1990

A dose-response analysis of ethylnitrosourea-induced recessive specific-locus mutations in treated spermatogonia of the mouse.

A dose-response analysis was carried out with 2 independent data sets available for ethylnitrosourea-induced specific-locus mutations in spermatogonia of the mouse. It was assumed that the occurrence of mutation is binomially distributed and maximum-likelihood procedures were employed to determine the appropriateness of 4 alternative models, Linear, Linear-Quadratic, Power, and Threshold, in describing the dependence of the binomial parameter on dose. For both data sets, the Threshold model yielded a far superior fit and the threshold dose was estimated to be between 34 and 39 mg/kg. These results are supported by the relatively inefficient response of ethylnitrosourea at lower doses in inducing DNA adducts. Relevant specific-locus mutation results in the mouse for low-dose fractionated treatment as well as the recovery of mutation mosaics indicate the threshold model to be an oversimplification. Rather than a threshold dose below which 100% of the induced DNA adducts are repaired, we propose that some DNA adducts which may eventually be fixed as a mutation persist through a number of repair-competent cell divisions and do not interfere with normal cell function nor do they induce a repair response before being eventually fixed as a mutation. We interpret the thresholded response for ethylnitrosourea-induced specific-locus mutations to be due to a saturable repair process which at lower doses results in ethylnitrosourea being less efficient in inducing mutation. Once this repair process is saturated, a clear dose-related increase in the mutation rate is observed.

Animals↗
PubMedDate not supplied

[Strategy of Ascaris lumbricoides and Trichuris trichiura for the environmental contamination, in an endemic area].

The distribution of frequencies of the number per grame of Ascaris lumbricoides and Trichuris trichiura in the faeces of naturally infected human in endemic areas, were found following a negative binomial distribution and an overdispersal pattern, independently of the ages of hosts. These results show, that only few hosts, independently of their ages, are involved in the higher environment contamination.

Adolescent↗
PubMed1998

Statistical Inference (part II): The Normal and Related Distributions.

The normal (or gaussian) is the most important probability distribution in the statistical inference process. By transforming the normal distribution to a standard distribution (z-distribution) it is possible to determine the probabilities where observations or values fall in certain intervals for variables with different means and standard deviations. Areas under the normal distribution may be represented in a table where the z is the number of standard deviations away from the mean. For example, the area under the normal distribution delimited by a z value of 1.96 to the left and 1.96 to the right of the mean corresponds to 95% of the total area of the normal distribution. Sampling distribution of estimates of population parameters may also be described by the normal distribution. It is important to note, however, that the estimation of a population mean based on the normal distribution is conditional to the assumption that the population standard deviation is a known parameter. The t distribution is used to infer about a population mean when the population standard deviation is estimated by the sample data. In statistical inference about proportions, the normal approximation of the binomial distribution may be used provided the data fit certain assumptions. Statistical methods that do not depend on the form of the distribution (distribution-free or nonparametric methods) and those based on the actual probability distribution, called exact methods, are often used in situations where the data do not comply with the assumption that the distribution of the estimate is approximately normal.

Journal Article↗