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At least 19 recordsLinked to original sources
PubMed1997

Map-based quantitative trait locus identification.

Poultry gene mappers chose microsatellites as the main source of genetic markers for poultry genome mapping, similar to the marker type used for other farm animals, laboratory animals, and humans. Optimal strategies for applying DNA markers in poultry populations are discussed, including the number of markers to be used, genome representation, population structure, choice of markers, population size, statistical stringency for association between markers and quantitative trait loci (QTL), and biological verification of a linkage. It is shown that an efficient strategy should be based on a combination of a low stringent statistical test for the existence of linkage between a marker and QTL and an appropriate genetic test for the discrimination between true and false linkage. The source of the genetic variation to be used is discussed and, as an illustration, three types of resource populations are presented. The informativeness of different matings using various genotypes of the parents are considered and it appears that selection of markers based on the heterozygosity of the sire is the most efficient marker screening approach.

Animals↗
PubMed1998

School size statistics of fish

A system of a fixed population size is considered in which fish schools break up and unite with other schools. The size distribution of schools is investigated on the basis of a balance equation, which corresponds to the mean-field theory of Smoluchowski-equation model of the coagulation-fragmentation process. The rates of fission and fusion are determined from a simple dynamic viewpoint of schooling. The size distribution, in effect, follows a power law up to a cutoff size, which can be fitted to data. The power index and the cutoff size depend on the population size. It is also elucidated how statistical properties of the system are regulated by the total population size. As the population size increases the number of schools increases, and asymptotically approaches a fixed value. If the population size is large, the mean school size depends linearly upon the population size. The standard deviation of the school-size distribution is proportional to the mean school size which is checked with data. Copyright 1998 Academic Press

Journal Article↗
PubMed1996

Evidence for the sensitivity of the SF-36 health status measure to inequalities in health: results from the Oxford healthy lifestyles survey.

OBJECTIVES: The short form 36 (SF-36) health questionnaire may not be appropriate for population surveys assessing health gain because of the low responsiveness (sensitivity to change) of domains on the measure. An hypothesised health gain of respondents in social class V to that of those in social class I indicated only marginal improvement in self reported health. Subgroup analysis, however, showed that the SF-36 would indicate dramatic changes if the health of social class V could be improved to that of social class I. DESIGN: Postal survey using a questionnaire booklet containing the SF-36 and a number of other items concerned with lifestyles and illness. A letter outlining the purpose of the study was included. SETTING: The sample was drawn from family health services authority (FHSA) computerised registers for Berkshire, Buckinghamshire, Northamptonshire, and Oxfordshire. SAMPLE: The questionnaire was sent to 13,042 randomly selected subjects between the ages of 17-65. Altogether 9332 (72%) responded. OUTCOME MEASURES: Scores for the eight dimensions of the SF-36. STATISTICS: The sensitivity of the SF-36 was tested by hypothesising that the scores of those in the bottom quartile of the SF-36 scores in class V could be improved to the level of the scores from the bottom quartile of SF-36 scores in class I using the effect size statistic. RESULTS: SF-36 scores for the population at the 25th, 50th, and 75th centiles were provided. Those who reported worse health on each dimension of the SF-36 (ie in the lowest 25% of scores) differ dramatically between social class I and V. Large effect sizes were gained on all but one dimension of the SF-36 when the health of those in the bottom quartile of the SF-36 scores in class V were hypothesised to have improved to the level of the scores from the bottom quartile of SF-36 scores in class I. CONCLUSIONS: Analysis of SF-36 data at a population level is inappropriate; subgroup analysis is more appropriate. The data suggest that if it were possible to improve the functioning and wellbeing of those in worst health in class V to those reporting the worst health in class I the improvement would be dramatic. Furthermore, differences between the classes detected by the SF-36 are substantial and more dramatic than might previously have been imagined.

Adolescent↗
PubMed1999

Increasing physicians' awareness of the impact of statistics on research outcomes: comparative power of the t-test and and Wilcoxon Rank-Sum test in small samples applied research.

To effectively evaluate medical literature, practicing physicians and medical researchers must understand the impact of statistical tests on research outcomes. Applying inefficient statistics not only increases the need for resources, but more importantly increases the probability of committing a Type I or Type II error. The t-test is one of the most prevalent tests used in the medical field and is the uniformally most powerful unbiased test (UMPU) under normal curve theory. But does it maintain its UMPU properties when assumptions of normality are violated? A Monte Carlo investigation evaluates the comparative power of the independent samples t-test and its nonparametric counterpart, the Wilcoxon Rank-Sum (WRS) test, to violations from population normality, using three commonly occurring distributions and small sample sizes. The t-test was more powerful under relatively symmetric distributions, although the magnitude of the differences was moderate. Under distributions with extreme skews, the WRS held large power advantages. When distributions consist of heavier tails or extreme skews, the WRS should be the test of choice. In turn, when population characteristics are unknown, the WRS is recommended, based on the magnitude of these power differences in extreme skews, and the modest variation in symmetric distributions.

Epidemiologic Studies↗
PubMed1997

Pharmacokinetic modelling of intravenous tobramycin in adolescent and adult patients with cystic fibrosis using the nonparametric expectation maximization (NPEM) algorithm.

The availability of personal computer programs for individualizing drug dosage regimens has stimulated the interest in modelling population pharmacokinetics. Data from 82 adolescent and adult patients with cystic fibrosis (CF) who were treated with intravenous tobramycin because of an exacerbation of their pulmonary infection were analysed with a non-parametric expectation maximization (NPEM) algorithm. This algorithm estimates the entire discrete joint probability density of the pharmacokinetic parameters. It also provides traditional parametric statistics such as the means, standard deviation, median, covariances and correlations among the various parameters. It also provides graphic-2- and 3-dimensional representations of the marginal densities of the parameters investigated. Several models for intravenous tobramycin in adolescent and adult patients with CF were compared. Covariates were total body weight (for the volume of distribution) and creatinine clearance (for the total body clearance and elimination rate). Because of lack of data on patients with poor renal function, restricted models with non-renal clearance and the non-renal elimination rate constant fixed at literature values of 0.15 L/h and 0.01 h-1 were also included. In this population, intravenous tobramycin could be best described by median (+/-dispersion factor) volume of distribution per unit of total body weight of 0.28 +/- 0.05 L/kg, elimination rate constant of 0.25 +/- 0.10 h-1 and elimination rate constant per unit of creatinine clearance of 0.0008 +/- 0.0009 h-1/(ml/min/1.73 m2). Analysis of populations of increasing size showed that using a restricted model with a non-renal elimination rate constant fixed at 0.01 h-1, a model based on a population of only 10 to 20 patients, contained parameter values similar to those of the entire population and, using the full model, a larger population (at least 40 patients) was needed.

Adolescent↗
PubMedDate not supplied

Pleural malignant mesothelioma and environmental asbestos exposure in Casale Monferrato, Piedmont. Preliminary analysis of a case-control study.

A case-control study on pleural malignant mesothelioma (MM) was conducted in Casale Monferrato, where the largest Italian asbestos cement (AC) factory had been operating from 1907 to 1985. In a previous study we observed a five to seven-fold increase in the incidence of MM among people living in that city and never employed in the factory mentioned. The present study includes cases of MM with histological diagnosis over the period 1.1.1987-30.6.1993 among residents in the Local Health Unit (LHU) of Casale Monferrato. Population controls were randomly extracted from the list of the residents in the LHU, matched to cases on sex, date of birth, vital status and date of death. Cases and controls (or their closest relative) were interviewed with a standardised questionnaire focusing on asbestos exposure in the (life-long) residential and occupational histories and in leisure time activities as well as on occupational asbestos exposure of relatives and cohabitants, smoking and chest or occupational diseases. The interview was blind in respect to case or control status. For the analyses the addresses were coded on map grids with a 500 m. mesh size. Statistical analyses were conducted with conditional logistic regression in order to keep the matching between cases and controls. Eighty-eight cases and 244 controls were interviewed (95.6% of cases and 80.1% of controls): 26 and 11 respectively reported an activity in the AC industry. Seven cases and 7 controls were also exposed because of parental occupation. The main analyses are based on the conditional regression model including both occupational and residential exposure. The different modes of exposure are included on an ordinal scale: each subject is classified according to their highest level. Domestic exposure is included as an independent factor. Odds Ratios (OR) are estimated with reference to subjects without either occupational or residential exposure. The OR is 39.3 among subjects reporting occupational exposure, 11.9 among those never engaged in the AC industry but living within 1000 m. of the factory. A statistically significant risk is also observed for persons at some time living in the other areas of Casale Monferrato.

Aged↗
PubMed1977

Quick test for comparing two populations with bivariate data.

A simple nonparametric test is given for testing the equality of two populations when the observations (X, Y) are bivariate and the alternative of interest is that the conditional c.d.f. of Y given X for one population dominates that for the other for every value of X. The procedure consists of fitting a curve through the combined set of data points and considering and conditional distribution of the number of points above the curve from one population, given certain information (depending on the design of the experiment), on the combined set of data. When used to compare two bivariate distributions, the test reduces to Fisher's exact test for 2 X 2 tables.

Animals↗
PubMed1997

Statistical and study design issues in assessing the quality and outcomes of care in rheumatic diseases.

As we have pointed out in this article, a health care study should have a well-defined intent that is matched to the study type, population of interest, and outcome. Care must be taken to collect samples in a meaningful way, so that results can be generalized to larger populations. Outcomes must be selected carefully to ensure that they will be sensitive to the types of care being considered, and only a few main outcomes should be selected so as to preserve the level of statistical significance of the research results. Sample sizes should be sufficient to detect an effect of reasonable size, after accounting for attrition in longitudinal studies and rates of occurrence with dichotomous outcomes. If the study purpose is to compare multiple institutions or health care providers, statistical adjustments for case mix will generally be required. Outcome research is an expanding area of development for rheumatology care and for medical care in general. It offers the promise of the use of administrative data bases to answer questions that are important both to arthritis researchers and to consumers of rheumatology care. As with all areas of clinical research, we must maintain appropriate levels of statistical rigor to protect the integrity of the results. Inadequate attention to the design and analysis of data can compromise research results before a study even gets started, and health care research studies have as many potential statistical pitfalls as other types of clinical research.

Epidemiologic Studies↗
PubMed1996

How interchangeable are different estimators of effect size?

The computation of effect sizes is a key feature of meta-analysis. In treatment outcome meta-analyses, the standardized mean difference statistic on posttest scores (d) is usually the effect size statistic used. However, when primary studies do not report the statistics needed to compute d, many methods for estimating d from other data have been developed. Little is known about the accuracy of these estimates, yet meta-analysts frequently use them on the assumption that they are estimating the same population parameter as d. This study investigates that assumption empirically. On a sample of 140 psychosocial treatment or prevention studies from a variety of areas, the present study shows that these estimates yield results that are often not equivalent to d in either mean or variance. The frequent mixing of d and other estimates of d in past meta-analyses, therefore, may have led to biased effect size estimates and inaccurate significance tests.

Electronic Data Processing↗
PubMed1998

A discordant-sibship test for disequilibrium and linkage: no need for parental data.

The sibship disequilibrium test (SDT) is designed to detect both linkage in the presence of association and association in the presence of linkage (linkage disequilibrium). The test does not require parental data but requires discordant sibships with at least one affected and one unaffected sibling. The SDT has many desirable properties: it uses all the siblings in the sibship; it remains valid if there are misclassifications of the affectation status; it does not detect spurious associations due to population stratification; asymptotically it has a chi2 distribution under the null hypothesis; and exact P values can be easily computed for a biallelic marker. We show how to extend the SDT to markers with multiple alleles and how to combine families with parents and data from discordant sibships. We discuss the power of the test by presenting sample-size calculations involving a complex disease model, and we present formulas for the asymptotic relative efficiency (which is approximately the ratio of sample sizes) between SDT and the transmission/disequilibrium test (TDT) for special family structures. For sib pairs, we compare the SDT to a test proposed both by Curtis and, independently, by Spielman and Ewens. We show that, for discordant sib pairs, the SDT has good power for testing linkage disequilibrium relative both to Curtis's tests and to the TDT using trios comprising an affected sib and its parents. With additional sibs, we show that the SDT can be more powerful than the TDT for testing linkage disequilibrium, especially for disease prevalence >.3.

Alleles↗
PubMed1998

The significance of non-significance.

We discuss the implications of empirical results that are statistically non-significant. Figures illustrate the interrelations among effect size, sample sizes and their dispersion, and the power of the experiment. All calculations (detailed in Appendix) are based on actual noncentral t-distributions, with no simplifying mathematical or statistical assumptions, and the contribution of each tail is determined separately. We emphasize the importance of reporting, wherever possible, the a priori power of a study so that the reader can see what the chances were of rejecting a null hypothesis that was false. As a practical alternative, we propose that non-significant inference be qualified by an estimate of the sample size that would be required in a subsequent experiment in order to attain an acceptable level of power under the assumption that the observed effect size in the sample is the same as the true effect size in the population; appropriate plots are provided for a power of 0.8. We also point out that successive outcomes of independent experiments each of which may not be statistically significant on its own, can be easily combined to give an overall p value that often turns out to be significant. And finally, in the event that the p value is high and the power sufficient, a non-significant result may stand and be published as such.

Analysis of Variance↗
PubMed1999

Statistical power of MRI monitored trials in multiple sclerosis: new data and comparison with previous results.

OBJECTIVES: To evaluate the durations of the follow up and the reference population sizes needed to achieve optimal and stable statistical powers for two period cross over and parallel group design clinical trials in multiple sclerosis, when using the numbers of new enhancing lesions and the numbers of active scans as end point variables. METHODS: The statistical power was calculated by means of computer simulations performed using MRI data obtained from 65 untreated relapsing-remitting or secondary progressive patients who were scanned monthly for 9 months. The statistical power was calculated for follow up durations of 2, 3, 6, and 9 months and for sample sizes of 40-100 patients for parallel group and of 20-80 patients for two period cross over design studies. The stability of the estimated powers was evaluated by applying the same procedure on random subsets of the original data. RESULTS: When using the number of new enhancing lesions as the end point, the statistical power increased for all the simulated treatment effects with the duration of the follow up until 3 months for the parallel group design and until 6 months for the two period cross over design. Using the number of active scans as the end point, the statistical power steadily increased until 6 months for the parallel group design and until 9 months for the two period cross over design. The power estimates in the present sample and the comparisons of these results with those obtained by previous studies with smaller patient cohorts suggest that statistical power is significantly overestimated when the size of the reference data set decreases for parallel group design studies or the duration of the follow up decreases for two period cross over studies. CONCLUSIONS: These results should be used to determine the duration of the follow up and the sample size needed when planning MRI monitored clinical trials in multiple sclerosis.

Adolescent↗
PubMed1992

[Detection of the safety depth on human chest by computer tomographic scanning].

Knowledges of acupuncture has played a major role in the treatment of human diseases, and has been mentioned in ancient Chinese medical literature for thousands of years. The loci which are most commonly used as treatment sites are points on the Ren Channel, kidney Channel, Stomach Channel, Pericardium Channel, Lung Channel, Spleen Channel and Gallbladder Channel for those loci on the human chest, insertion depth beyond safety level, can create serious consequences other such as pneumothorax, internal organ bleeding or other damage. This study was designed to determine the real safety depth for each locus by using a current scientific approach. The study was carried out at Tri-Service General Hospital and Mackay General Hospital. The sample patient population was 120. According to their body weight and height, patients were divided into six groups with various body sizes, i.e. normal, over and under-weight adults, and by sex differences. After computer tomographic scanning chest, the relative acupuncture loci should be measured according to anatomical position, then the distance between surface of the chest and thoracic pleura can be defined as its safety depth. For each locus of the groups, the mean and its interval of confident can be found. The analysis of variance (ANOVA), t-test, and multiple regression were also calculated by computer. The results show that there are significant differences in body chest loci within the same sex but, for different body sizes, statistically significant differences for each locus appear, so the safety depth for each chest locus has actually been proved. Not only does his data provide useful information for clinical practices, but also the standard safety depth for each loci on the chest could this be established.

Acupuncture Points↗