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PubMed2000

Tests for homogeneity of the risk ratio in a series of 2x2 tables.

We often apply the risk ratio to measure the strength of a causal relationship between a suspected risk factor and a disease of interest. In this paper we consider testing the homogeneity of risk ratio over a series of 2x2 tables. In addition to the classical weighted least squares (CWLS) test procedure, we consider two test procedures using simple transformations of the CWLS statistic and develop three other asymptotically weighted test procedures. On the basis of Monte Carlo simulation, we conclude that the commonly-used CWLS test procedure is generally conservative, especially when the number of 2x2 tables is large and the mean group size per table is moderate or small. We further find that two of the test procedures discussed here can not only generally outperform the CWLS test procedure, but also perform well in a variety of situations considered in this paper. Finally, we illustrate the use of these testing procedures with an example of six randomized trials that assess the effects of aspirin on the prevention of death in post-myocardial infarction patients.

Aspirin↗
PubMed2000

Exact sequential tests for single samples of discrete responses using spending functions.

Sequential tests are increasingly used to reduce the expected sample size of trials in medical research. The majority of such methods are based on the assumption of normality for test statistics. In clinical trials yielding a single sample of discrete data, that assumption is often poorly satisfied. In this paper we show how a novel application of the spending function approach of Lan and DeMets can be used together with exact calculation methods to design sequential procedures for a single sample of discrete random variables without the assumption of normality. A special case is that of binomial data and the paper is illustrated by the design of a cytogenetic study which motivated this work.

Binomial Distribution↗
PubMed2000

Estimates of use and costs of behavioural health care: a comparison of standard and finite mixture models.

Estimates of health care demand are known to depend on the empirical specification used in the analysis. In this paper, an innovative specification, the finite mixture model (FMM), is employed to estimate the utilization of and expenditures on behavioural health care. Unlike standard specifications, the FMM has the ability to distinguish between distinct classes of users of behavioural health care (e.g. the 'worried well' and the severely mentally ill). This new model is tested against standard empirical specifications using data from the National Medical Expenditure Survey. Using common risk stratifiers, estimates of utilization and costs are generated with each specification. It is found that the FMM provides a much better fit of both expenditure and utilization data than standard specifications, particularly among high intensity users that standard models have been unable to represent adequately. Furthermore, the results provide preliminary evidence that there are (at least) two distinct groups of users of behavioural health care. The empirical advantages of the FMM translate into superior estimates of mean costs and utilization that have widespread application in rate-setting exercises.

Adolescent↗
PubMed2000

Health services utilization of elderly Swiss: evidence from panel data.

The demand for health care services by the elderly is a topic of growing importance because of changes in the demographic structure in many countries. This paper provides estimates of the determinants of the demand for physician visits by the elderly, including the impact of a disability prevention intervention. We control for unobserved heterogeneity across individuals and the count data structure of the data by estimating random effects negative binomial models for all primary physician and specialist visits.

Aged↗
PubMed1994

Sources of variability in data from a lacI transgenic mouse mutation assay.

Experimental features of a transgenic mouse mutation assay based on a lacI target transgene from Escherichia coli are considered in detail. Sources of variability in the experimental protocol that can affect the statistical nature of the observations are examined with the goal of identifying sources of excess variation in the observed mutant fractions. The sources include plate-to-plate (within packages), package-to-package (within animals), and animal-to-animal (within study) variability. Data from two laboratories are evaluated, using various statistical methods to identify excess variability. Results suggest only scattered patterns of excess variability, except possibly in those cases where genomic DNA from test animals is stored for extended periods (e.g., > 90 days) after isolation from tissues. Further study is encouraged to examine the validity and implications of this time/storage-related effect.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1994

Chemical mutagenesis testing in Drosophila. X. Results of 70 coded chemicals tested for the National Toxicology Program.

Seventy chemicals were tested for the ability to induce sex-linked recessive lethal (SLRL) mutations in postmeiotic and meiotic germ cells of male Drosophila melanogaster. As in the previous studies in this series, adult feeding was chosen as the first route of administration. If the compound failed to induce mutations by this route, injection exposure was used. Two chemicals, n-butane and propylene, were gaseous and therefore tested only by inhalation. One chemical (dimethylcarbamoyl chloride) was tested only by injection. Those chemicals that were mutagenic in the SLRL assay were further tested for the ability to induce reciprocal translocations. Sixteen of the 70 chemicals tested were mutagenic in the SLRL assay: 3-chloro-2-methylpropene, 3-(chloromethyl)pyridine HCl, dimethylcarbamoyl chloride, HC blue 1,3-iodo-1,2-propanediol, malaoxon, N,N'-methylene-bis-acrylamide, 4,4'-methylenedianiline 2HCl, ziram, cis-dichlorodiaminoplatinum II, 1,2-dibromoethane, dibromomannitol, 1,2-epoxypropane, glycidol, myleran, and toluene diisocyanate. The last seven also induced reciprocal translocations. A comparison of the results from the SLRL assay with other assays for mutagens and carcinogens suggests that the SLRL assay is highly specific, but poorly sensitive, both for mutagens and potential carcinogens.

Animals↗
PubMed1995

Study design and sample sizes for a lacI transgenic mouse mutation assay.

Design features that adjust and account for excess variation in a transgenic mouse mutation assay based on a lacI target transgene from E. coli are considered. These features include proper identification of plate, packaging reaction, and animal identifier codes throughout the experimental and analysis phases of the study, "blocking" of exposed and unexposed animals when preparing and plating multiple packaging reactions from the same genomic DNA sample, separating sectored mutant plaques and complete mutant plaques before performing any quantitative analyses, and testing for sources of excess variation attributable to features of the experimental protocol--such as plate-to-plate (within packaging reactions), packaging reaction-to-packaging reaction (within animals), and animal-to-animal (within study). Control and ethylnitrosourea-treated animal data are presented from a fully designed study in the lacI assay. The study design incorporates many of these experimental principles. Statistical methods to identify excess variability are noted, and the designed study data are used to illustrate the types of variability encountered in practice. A standard statistical test for two-sample testing is highlighted, from which recommendations are made for sample size selection in future studies.

Analysis of Variance↗
PubMed1995

Statistical design and analysis of mutation studies in transgenic mice.

We have been working on identifying sources of variability in data from transgenic mouse mutation assays in order to develop appropriate statistical methods and designs for routine studies. Data from our lab and elsewhere point to the presence of significant animal-to-animal variability, which must be taken into account in statistical hypothesis tests. Here, the usual Cochran-Armitage (CA) test for trend in mutant frequencies, which takes the transgene as the experimental unit, and a generalized Cochran-Armitage test (GCA), which takes the animal as the experimental unit, are contrasted in computer simulations that help to quantify the differences between these statistical tests. The simulations report the statistical power of each test to detect treatment group differences, and their type I error rates. We find in general that the GCA test performs poorly compared to the CA test when it is appropriate to take the transgene as the experimental unit, and the study also uses a small number of animals. However, the CA test performs poorly in small group-size studies when the animal is the appropriate experimental unit. Extensions of the computer simulations allow for identification of cost-effective experimental designs. The results emphasize that the benefits of using additional animals in these mutation studies can be realized without substantial increases in costs. Here we illustrate the methods for liver studies in our lab. These methods can be used to derive optimal experimental designs for any combination of spontaneous mutant frequency and animal-to-animal variability.

Animals↗
PubMed1994

Genetic analysis combining path analysis with regressive models: the BETA path model of polygenic and familial environmental transmission.

We have extended the class D regressive model for the purpose of combined path and segregation analyses by incorporating the BETA path model. We have done this by expressing correlations among residuals from major genotype (RMGs) of family members under the class D regressive model as functions of path coefficients under the BETA path model. The likelihood function under the combined model was factorized into a product of conditional densities, which is dominated by bivariate normal densities. Statistical inferences under the combined model are analogous to those under the class D regressive model.

Binomial Distribution↗
PubMed2006

De novo chromosomal abnormalities and month of conception.

OBJECTIVES: We set out to study the possible relationship between the occurrence of chromosomal abnormalities and the month of conception using data from 7 years of prenatal diagnosis. METHODS: The sample included 7439 cytogenetic analyses from amniocentesis performed for conceptions between 1997 and 2003. The monthly prevalence of all de novo chromosomal abnormalities, trisomies, including trisomy 21, and Robertsonian translocations was investigated. RESULTS: Two hundred and four de novo numerical and structural chromosomal abnormalities (2.7%) were detected. A significant decrease in the frequencies of all de novo chromosomal abnormalities for June conceptions, as well as lower summer occurrences of trisomy 21, were found. In addition, de novo Robertsonian translocations were unexpectedly detected only among December conceptions. CONCLUSIONS: The relationship between chromosomal abnormalities and the month of conception could turn out to be useful in estimating the actual risk in pregnancy, if confirmed by other studies. The observed June lowest rate might indicate an association between chromosomal abnormalities and the maximum number of daylight hours throughout the year (summer solstice). We cannot explain the unusual findings concerning Robertsonian translocations that were only found for conceptions in December. This may be related to the annual minimum daylight hours (winter solstice) as opposed to the summer solstice.

Amniocentesis↗
PubMed1992

Correcting for the Bernoulli effect in lateral pressure measurements.

Measurement of airway pressure is essential in the study of respiratory mechanics, and is usually done via a lateral tap in the conduit (e.g., endotracheal tube, cannula, or mouthpiece) leading into the subject's airway. Such pressure measurements, however, may be severely affected by the Bernoulli effect if the diameter of the conduit is small and the gas flow through it sufficiently high. We present in this note a simple method of assessing whether or not the Bernoulli effect is important in any particular situation. The technique involves comparing the pressure-flow relationships of the conduit obtained both by blowing air through it from one end and sucking air in the reverse direction by applying negative pressure at the same end. If the resistance of the conduit is the same for gas flow in both directions, then half the magnitude of the difference between the two pressure-flow relationships gives the magnitude of the Bernoulli effect pressure. We give results of an experimental situation in which this was the case. We also show that those conditions under which the Bernoulli effect is likely to be a problem are also those under which the velocity profile is likely to be approximately flat, thereby permitting the magnitude of the Bernoulli effect to be easily calculated.

Air Pressure↗
PubMed2002

A graphical sensitivity analysis for clinical trials with non-ignorable missing binary outcome.

Many clinical trials are analysed using an intention-to-treat (ITT) approach. A full application of the ITT approach is only possible when complete outcome data are available for all randomized subjects. In a recent survey of clinical trial reports including an ITT analysis, complete case analysis (excluding all patients with a missing response) was common. This does not comply with the basic principles of ITT since not all randomized subjects are included in the analysis. Analyses of data with missing values are based on untestable assumptions, and so sensitivity analysis presenting a range of estimates under alternative assumptions about the missing-data mechanism is recommended. For binary outcome, extreme case analysis has been suggested as a simple form of sensitivity analysis, but this is rarely conclusive. A graphical sensitivity analysis is proposed which displays the results of all possible allocations of cases with missing binary outcome. Extension to allow binomial variation in outcome is also considered. The display is based on easily interpretable parameters and allows informal examination of the effects of varying prior beliefs.

Angioplasty, Balloon, Coronary↗
PubMed2003

Confidence intervals for the binomial parameter: some new considerations.

Several methods have been proposed to construct confidence intervals for the binomial parameter. Some recent papers introduced the "mean coverage" criterion to evaluate the performance of confidence intervals and suggested that exact methods, because of their conservatism, are less useful than asymptotic ones. In these studies, however, exact intervals were always represented by the Clopper-Pearson interval (C-P). Now we focus on Sterne's interval, which is also exact and known to be better than the C-P in the two-sided case. Introducing a computer intensive level-adjustment procedure which allows constructing intervals that are exact in terms of mean coverage, we demonstrate that Sterne's interval performs better than the best asymptotic intervals, even in the mean coverage context. Level adjustment improves the C-P as well, which, with an appropriate level adjustment, becomes equivalent to the mid-P interval. Finally we show that the asymptotic behaviour of the mid-P method is far poorer than is generally expected.

Binomial Distribution↗
PubMed2003

Truncated negative binomial mixed regression modelling of ischaemic stroke hospitalizations.

A zero-truncated negative binomial mixed regression model is presented to analyse overdispersed positive count data. The study is motivated by the determination of pertinent risk factors associated with ischaemic stroke hospitalizations. Random effects are incorporated in the linear predictor to adjust for inter-hospital variations and the dependency of clustered observations using the generalized linear mixed model approach. The method assists hospital administrators and clinicians to estimate the number of subsequent readmissions based on characteristics of the patient at the index stroke. The findings have important implications on resource usage, rehabilitation planning and management of acute stroke care.

Binomial Distribution↗
PubMed2005

Bayesian tests of extra-Binomial variability.

A simple model for extra-Binomial variability is the Beta-Binomial. A complication in testing the Binomial against the Beta-Binomial alternative is that the Binomial lies on the boundary of the Beta-Binomial, which forces modifications to the usual asymptotic arguments. In this paper, we propose a Bayesian test using a pair of approximate Bayes factors, one for the case in which the maximum likelihood estimator (MLE) of the extra-Binomial variability is zero and one for the case in which it is positive. These approximate Bayes factors are easy to compute. We evaluate the operating characteristics of the Bayes factors and find them to be more powerful than the likelihood ratio test. We then apply the method to three data sets, including one in which the issue is whether a logistic regression intercept should be considered a random effect. In each case, our approximate Bayes factors are close to the exact Bayes factors, which may also be computed with additional effort.

Amputation, Surgical↗
PubMed2004

Sensitivity of score tests for zero-inflation in count data.

In many biomedical applications, count data have a large proportion of zeros and the zero-inflated Poisson regression (ZIP) model may be appropriate. A popular score test for zero-inflation, comparing the ZIP model to a standard Poisson regression model, was given by van den Broek. Similarly, for count data that exhibit extra zeros and are simultaneously overdispersed, a score test for testing the ZIP model against a zero-inflated negative binomial alternative was proposed by Ridout, Hinde and Demétrio. However, these test statistics are sensitive to anomalous cases in the data, and incorrect inferences concerning the choice of model may be drawn. In this paper, diagnostic measures are derived to assess the influence of observations on the score statistics. Two examples that motivated the application of zero-inflated regression models are considered to illustrate the importance of sensitivity analysis of the zero-inflation tests.

Accidents, Occupational↗
PubMed2005

Evaluating current policy for detecting mosaicism in amniotic fluid cultures: implications for current cell counting practices.

Chromosomal mosaicism is one of the most vexing problems for clinical cytogenetic laboratories and personnel time used for analysis at the microscope is one of the principle costs in cytogenetic laboratories. We use data collected from 26 cytogenetic laboratories to evaluate whether the American College of Medical Genetics guidelines for minimum number of cells to count to exclude mosaicism in amniotic fluid specimens is appropriate. An accurate estimate of the number of mosaics that are missed by current cell counting practices is an important step in this process. Thus, we present a new method for estimating the number of mosaics that are missed and we use computer simulation to evaluate this new method. Our results indicate that if the clinical significance of mosaicism is suspected to be minimal for certain cytogenetic anomalies when the percentage of abnormal cells is 15 per cent or less, then it may be sufficient to use a 15-cell counting-rule-for-detection along with a minimum total cell count of 30 regardless of whether abnormal cells or normal cells are in the minority.

Amniotic Fluid↗