Biomedical subjects

W Y Poon

Publications and source records attributed to W Y Poon.

7 recordsLinked to original sources

PubMed2001

Conditional local influence in case-weights linear regression.

The local influence approach proposed by Cook (1986) makes use of the normal curvature and the direction achieving the maximum curvature to assess the local influence of minor perturbation of statistical models. When the approach is applied to the linear regression model, the result provides information concerning the data structure different from that contributed by Cook's distance. One of the main advantages of the local influence approach is its ability to handle the simultaneous effect of several cases, namely, the ability to address the problem of 'masking'. However, Lawrance (1995) points out that there are two notions of 'masking' effects, the joint influence and the conditional influence, which are distinct in nature. The normal curvature and the direction of maximum curvature are capable of addressing effects under the category of joint influences but not conditional influences. We construct a new measure to define and detect conditional local influences and use the linear regression model for illustration. Several reported data sets are used to demonstrate that new information can be revealed by this proposed measure.

Humans↗

PubMed2000

A local influence approach to identifying multiple multivariate outliers.

We make use of Cook's local influence approach and its recent modification by Poon and Poon to develop measures for detecting multivariate outliers. The motivation and the foundation of the theory are geometrical and are different from classical approaches; however, whilst the proposed measure exhibits a form similar to those in the literature, it still has a considerable advantage in having transformed the classical measures to the unit interval. The new approach unifies outlier identification measures using geometrical concepts. It involves no distributional assumption or large-sample properties, and allows the flexibility of identifying outliers with respect to different metrics. The approach therefore provides a valid reason for using the various measures in complicated situations, such as in non-normal cases and in small-sample problems.

Humans↗

PubMed1999

Bayesian analysis of square ordinal-ordinal tables.

We analyse square contingency tables with ordered categories. Assuming that the observed ordinal categorical variables are manifestations of underlying continuous variables, we formulate a model which allows the comparisons of locations and dispersions between variables. We identify the model by imposing stochastic constraints on the thresholds that define the relationship between the observed and the underlying variables. As a result, the underlying continuous variables' location and dispersion parameters which were not estimable before can be estimated by the Bayesian approach. Illustrative examples are given based on several reported data sets.

Bayes Theorem↗

PubMed1995

A two-stage estimation of structural equation models with continuous and polytomous variables.

This paper develops a computationally efficient procedure for analysis of structural equation models with continuous and polytomous variables. A partition maximum likelihood approach is used to obtain the first stage estimates of the thresholds and the polyserial and polychoric correlations in the underlying correlation matrix. Then, based on the joint asymptotic distribution of the first stage estimator and an appropriate weight matrix, a generalized least squares approach is employed to estimate the structural parameters in the correlation structure. Asymptotic properties of the estimators are derived. Some simulation studies are conducted to study the empirical behaviours and robustness of the procedure, and compare it with some existing methods.

Humans↗

PubMed1992

Two-level analysis of covariance structures for unbalanced designs with small level-one samples.

The main purpose of this paper is to develop basic statistical theory for two-level analysis of covariance structures. Major asymptotic results for statistical inference, such as the asymptotic distributions of the estimator and the goodness-of-fit test statistic are derived, based on an unbalanced design with only small numbers of level-one units. Computationally, it is shown that the solution can be obtained via standard programs, such as LISREL, EQS and COSAN. The behaviour of the estimates is illustrated by an artificial example and a real-life example. Some possibilities of extending the results to a more general two-level model, and to situations with arbitrary distributions and elliptical distributions are also investigated.

Analysis of Variance↗

PubMed1990

A two-phase linear regression model for biologic half-life data.

In estimating the biologic half-life of an infused drug or biologic agent, one very frequently used model is the biexponential, which reflects a two-compartment physiologic model. The difficulty in using this model is that it is nonlinear in the parameters and requires relatively sophisticated analysis. Furthermore, an interval estimate is not usually reported. We propose the use of a two-phase linear regression approach, which is tantamount to breaking down the model into two straight lines based on the selection of the logarithm of concentration as the ordinate and time as the abscissa. We show how to determine the joint or changeover point for the two lines using a simple iterative procedure, how to select between a one-phase and a two-phase model, and how to provide a simple confidence interval estimate for half-life when it exists. An example using data from a study of Factor VIII pharmacokinetics is given.

Factor VIII↗

PubMed1978

In vivo changes in the composition of cattle skin surface lipid with time.

The skin surface lipid composition was examined in cattle over a period of 13 days after cleaning the skin at 15 degrees C and 35 degrees C. Little change was observed in the concentrations and fatty acid compositions of all the major sebum fractions other than the unesterified fatty acids. Changes which occurred in the proportions of fatty acids present in the unesterified fatty acid fraction were similar at both environments. At 15 degrees C the total amounts per unit area of unesterified fatty acids on the skin did not alter significantly with time. However, at 35 degrees C there was a significant increase in the total amounts of unesterified fatty acids per unit area and concomitant increases in the amounts of individual fatty acids, in particular linoleic acid.

Animals↗