Biomedical subjects

Mary E Thompson

Publications and source records attributed to Mary E Thompson.

5 recordsLinked to original sources

PubMed2004

Modeling event times with multiple outcomes using the Wiener process with drift.

Length of stay in hospital (LOS) is a widely used outcome measure in Health Services research, often acting as a surrogate for resource consumption or as a measure of efficiency. The distribution of LOS is typically highly skewed, with a few large observations. An interesting feature is the presence of multiple outcomes (e.g. healthy discharge, death in hospital, transfer to another institution). Health Services researchers are interested in modeling the dependence of LOS on covariates, often using administrative data collected for other purposes, such as calculating fees for doctors. Even after all available covariates have been included in the model, unexplained heterogeneity usually remains. In this article, we develop a parametric regression model for LOS that addresses these features. The model is based on the time, T, that a Wiener process with drift (representing an unobserved health level process) hits one of two barriers, one representing healthy discharge and the other death in hospital. Our approach to analyzing event times has many parallels with competing risks analysis (Kalbfleisch and Prentice, The Statistical Analysis of Failure Time Data, New York: John Wiley and Sons, 1980)), and can be seen as a way of formalizing a competing risks situation. The density of T is an infinite series, and we outline a proof that the density and its derivatives are absolutely and uniformly convergent, and regularity conditions are satisfied. Expressions for the expected value of T, the conditional expectation of T given outcome, and the probability of each outcome are available in terms of model parameters. The proposed regression model uses an approximation to the density formed by truncating the series, and its parameters are estimated by maximum likelihood. An extension to allow a third outcome (e.g. transfers out of hospital) is discussed, as well as a mixture model that addresses the issue of unexplained heterogeneity. The model is illustrated using administrative data.

Health Services Research↗

PubMed2004

Estimation of fish abundance indices based on scientific research trawl surveys.

The fish abundance index over an ocean region is defined here to be the integral of expected catch per unit effort (CPUE), approximated by the sum of expected CPUE over grid squares. When trawl surveys are done within grid squares selected according to a probability sampling design, several other sources of variation such as the fish population dynamics and the catching process are also involved. In such situations model-assisted methods for estimating abundance, assessed under both design and model perspectives, have some advantages over purely design-based methods such as the Horvitz-Thompson (HT) estimator or purely model-based prediction approaches. This article develops model-assisted empirical likelihood (EL) methods via loglinear regression and nonparametric smoothing. The methods are applied to grid surveys of the Grand Bank region carried out annually by Fishery Products International from 1996 through 2002. The HT and EL methods produce similar point estimates of abundance indices. Simulation results, however, indicate that the EL estimator under local linear smoothing is associated with smaller standard errors.

Animals↗

PubMed2004

Berg Balance Scale and Functional Reach: determining the best clinical tool for individuals post acute stroke.

OBJECTIVE: To begin the process of determining the best clinical tool for measuring balance in individuals post stroke. DESIGN: Prospective with descriptive and relational analyses. SETTING: Rehabilitation centres and stroke support group meetings. SUBJECTS: Seventy-five subjects who had suffered a stroke were divided into four groups based on the Duncan classification for stroke severity. MAIN OUTCOME MEASURES: The motor section of the Fugl-Meyer Assessment of Physical Performance was completed for each person to classify the motor deficit. Subjects performed Functional Reach twice with the nonparetic arm and the average was recorded. Subjects performed the Berg Balance Scale and the total score on the first attempt was recorded. RESULTS: As motor impairment increased, balance ability declined and both the Functional Reach and Berg Balance Scale proved sensitive to this decline. Spearman rank correlation coefficient was excellent among the entire sample (r(s) =0.78). CONCLUSION: Subjects' performance on the Berg Balance Scale was closely associated with performance on the Functional Reach. Therefore, the clinician may elect to use the shorter Functional Reach as a measure of balance where efficient use of time is the primary goal.

Humans↗

PubMed2003

Bias correction of two-state latent Markov process parameter estimates under misclassification.

A discretely observed two-state process may misclassify the state of an unobservable continuous-time, two-state Markov process. We examine the behaviour of maximum likelihood transition probability estimates as functions of known misclassification probabilities. Since maximum likelihood estimators are not available in closed form, we provide two alternatives for bias-adjusted estimation. In the case of large samples, the asymptotic bias is quantified and estimators are constructed iteratively using transition counts and specified misclassification probabilities. For finite samples, we provide an approximation based on partial derivatives. Estimators that are bias-adjusted to a first approximation are easily constructed and may serve well when misclassification probabilities are known to be small. Simulation studies reveal the effect of misclassification on estimation. Repeated diagnostic testing data illustrate the approaches.

Animals↗

PubMed2003

Adenocarcinoma in situ arising from the submucosal oesophageal mucous glands.

The incidence of adenocarcinoma of the oesophagus is increasing; this type of carcinoma commonly arises on Barrett's oesophagus. We report a case of in-situ adenocarcinoma of the lower oesophagus arising in submucosal oesophageal mucous glands without intestinal metaplasia. We describe the histological findings, discuss the difficulties of differentiating this from invasive disease and review the current literature regarding this rare condition.

Adenocarcinoma, Mucinous↗