PubMed Health⌕ Search

Biomedical subjects

A R Sampson

Publications and source records attributed to A R Sampson.

9 recordsLinked to original sources
PubMed1999

A regression model for multivariate random length data.

Multivariate random length data occur when we observe multiple measurements of a quantitative variable and the variable number of these measurements is also an observed outcome for each experimental unit. For example, for a patient with coronary artery disease, we may observe a number of lesions in that patient's coronary arteries, along with percentage of blockage of each lesion. Barnhart and Sampson first proposed the multiple population model to analyse multivariate random length data without covariates. This paper extends their approach to deal with multiple covariates. We propose a new multiple population regression model with covariates, and discuss the estimation issues. We analyse data from the TYPE II coronary intervention study to illustrate the methodology.

Cholestyramine Resin↗
PubMed1999

Lamina-specific alterations in the dopamine innervation of the prefrontal cortex in schizophrenic subjects.

OBJECTIVE: Abnormalities in dopamine neurotransmission in the prefrontal cortex have been implicated in the pathophysiology of schizophrenia. However, the integrity of the dopamine projections to the prefrontal cortex in this disorder has not been directly examined. METHOD: The authors employed immunocytochemical methods and antibodies against tyrosine hydroxylase, the rate-limiting enzyme in dopamine biosynthesis, and the dopamine membrane transporter to examine dopamine axons in the dorsomedial prefrontal cortex (area 9) from 16 pairs of schizophrenic and matched control subjects. RESULTS: Compared to the control subjects, the total length of tyrosine hydroxylase-immunoreactive axons was unchanged in the superficial and middle layers of the schizophrenic subjects but was reduced by an average of 33.6% in layer 6. The total length of tyrosine hydroxylase-positive axons in layer 6 was decreased in 13 of the schizophrenic subjects compared to their control subjects. Axons immunoreactive for the dopamine membrane transporter showed a similar pattern of change. In contrast, axons labeled for the serotonin transporter did not differ between schizophrenic and control subjects in any layer examined. In addition, the density of tyrosine hydroxylase-containing axons did not differ between monkeys chronically treated with haloperidol and matched control animals. CONCLUSIONS: These findings reveal that schizophrenia is associated with an altered dopamine innervation of prefrontal cortex area 9 that is lamina- and neurotransmitter-specific and that does not appear to be a consequence of pharmacological treatment. Together, these data provide direct evidence for a disturbance in dopamine neurotransmission in the prefrontal cortex of schizophrenic subjects.

Adult↗
PubMed1999

The predictive power of baseline hemoglobin for transfusion risk in surgery patients.

Preoperative hemoglobin concentration may be an important predictor of transfusion risk in surgical procedures with significant expected blood loss. Contemporary studies investigating transfusion risk with regard to the relationship between perioperative administration of Epoetin alfa and baseline hemoglobin provide data to test this hypothesis. The predictive power of seven preoperative variables (hemoglobin concentration, age, erythropoietin level, ferritin concentration, serum iron, total iron-binding capacity, and predicted blood volume) on transfusion risk was examined via retrospective logistic regression analysis of 276 orthopedic surgical patients. In the two studies used to perform the regression analysis, patients were treated daily with either Epoetin alfa or placebo. Based on the retrospective analyses, a prospective study was conducted to validate the hypothesis. Of the seven variables evaluated, baseline hemoglobin concentration and predicted blood volume were significantly predictive of transfusion risk in both Epoetin alfa- and placebo-treated patients. Further, an inverse correlation between hemoglobin concentration and transfusion risk was demonstrated in placebo-treated patients. Placebo-treated patients with hemoglobin > 10 to < or = 13 g/dL had an approximately twofold greater risk of transfusion than patients with hemoglobin > 13 g/dL. In contrast to placebo treatment, Epoetin alfa significantly reduced transfusion risk in patients with hemoglobin > 10 to < or = 13 g/dL. Baseline hemoglobin concentration is an excellent predictor of transfusion risk in orthopedic surgical patients. As a result, hemoglobin testing should be considered a part of routine preoperative testing for orthopedic surgical patients.

Arthroplasty, Replacement, Hip↗
PubMed1995

Effects of heteroscedasticity upon certain analyses when regression lines are not parallel.

The two sample analysis of covariance model is considered where the regression lines are found not to be parallel. There are two analyses often utilized in this case. One is to obtain a standard confidence interval for the covariate value where the two lines intersect, and the other is to find a simultaneous confidence region for the difference between the regression lines. The asymptotic robustness to heteroscedasticity of the regressions' errors of the coverage probability of each of these two confidence procedures is considered. When the sample sizes are approximately equal, and the covariate means and the covariate variances are similar between the samples, the unequal regression variances are shown to have little effect on the asymptotic coverage probabilities of these two confidence procedures. Discussions concerning the effects of unequal sample sizes and inequitably distributed covariate values are also presented. These results are illustrated in application to a clinical trial of recombinant human erythropoetin versus placebo to treat patients with anemia secondary to advanced cancer, who were not receiving chemotherapy.

Analysis of Variance↗
PubMed1995

Multiple population models for multivariate random length data--with applications in clinical trials.

This paper focuses on the development and study of multiple population models for multivariate random length data, of the type often encountered in clinical trials. If experimental outcomes per subject consist of multiple measurements of a quantitative variable and the number of these measurements, then a multivariate random length vector is observed. For this type of data, the experimental treatment is likely to affect both the quantitative measurements and the number of these measurements. One example of such data is from the National Heart, Lung and Blood Institute Type II coronary intervention study (Brensike et al. (1982; Controlled Clinical Trials 3, 91-111; 1984, Circulation 69, 313-324)). The outcome data consist of vectors of lesion sizes with lengths determined by the number of underlying lesions assessed from the patients' angiograms, where both the numbers and the lesion sizes depend on patients' overall disease status. We propose models which can realistically describe the relationships between the quantitative variables and the number of responses. The asymptotic covariance of the maximum likelihood estimators is obtained. Data from the Type II study are analyzed using this multiple population model.

Biometry↗
PubMed1989

Multiple correlation: exact power and sample size calculations.

This article discusses power and sample size calculations for observational studies in which the values of the independent variables cannot be fixed in advance but are themselves outcomes of the study. It reviews the mathematical framework applicable when a multivariate normal distribution can be assumed and describes a method for calculating exact power and sample sizes using a series expansion for the distribution of the multiple correlation coefficient. A table of exact sample sizes for level .05 tests is provided. Approximations to the exact power are discussed, most notably those of Cohen (1977). A rigorous justification of Cohen's approximations is given. Comparisons with exact answers show that the approximations are quite accurate in many situations of practical interest. More extensive tables and a computer program for exact calculations can be obtained from the authors.

Computer Simulation↗
PubMed1988

A statistical methodology for postmarketing surveillance of adverse drug reaction reports.

This paper presents a statistically optimal exact hypothesis testing procedure for detecting changes in sales adjusted adverse drug reaction (ADR) rates between historical and current periods, with a computer program that implements this test appended. We provide discussions and illustrations on how to monitor ADR rates for product lines that consist of several pharmacologically equivalent dosage forms.

Computer Simulation↗
PubMedDate not supplied

Efficacy and safety of becaplermin (recombinant human platelet-derived growth factor-BB) in patients with nonhealing, lower extremity diabetic ulcers: a combined analysis of four randomized studies.

The results of a combined analysis and separate analyses of four multicenter, randomized, parallel group studies that evaluated the effects of once-daily topical administration of becaplermin gel for the treatment of chronic, full thickness, lower extremity diabetic ulcers are presented. The four studies included a total of 922 patients with nonhealing lower extremity diabetic ulcers of at least 8 weeks' duration. Following initial complete sharp debridement of the ulcer, patients were randomized to receive a standardized regimen of good ulcer care alone, good ulcer care plus placebo gel, or good ulcer care plus becaplermin gel-30 microg/g, or good ulcer care plus becaplermin gel-100 microg/g, with various combinations of regimens used in the four studies. Safety was assessed by monitoring adverse events and by clinical laboratory evaluations. Meta-analytic statistical techniques were used in the combined analysis to establish homogeneity of treatment comparisons across studies. Based on an analysis of patients with baseline ulcer area common to all trials (</= 10 cm2), representing 95% of all patients, becaplermin gel-100 microg/g significantly increased (p = 0.007) the probability of complete healing compared with placebo gel. It was determined that for the median ulcer area of these patients, which was 1.5 cm2, the becaplermin gel-100 microg/g treatment group showed a 39% increase in complete healing compared with that of the placebo gel treatment group (50% vs. 36%, respectively, p = 0.007). Becaplermin gel-100 microg/g significantly decreased (p = 0.01) the time to complete healing compared with placebo gel, with the 35th percentile of time to complete healing being reduced by 30% (14.1 weeks vs. 20. 1 weeks, respectively). In patients with ulcers </= 5 cm2 at baseline (a more homogeneous group), becaplermin gel-100 microg/g also significantly increased the incidence of complete healing with a similar decrease in the time to healing. Adverse events reported during treatment or during a 3-month follow-up period were not unexpected for this patient population and were similar in nature and incidence across all treatment groups. We therefore conclude that treatment with becaplermin gel at a dose of 100 microg/g once daily, in conjunction with good ulcer care, is effective and well tolerated in patients with full thickness lower extremity diabetic ulcers.

Administration, Topical↗