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S D Walter

Publications and source records attributed to S D Walter.

At least 91 records · Page 5Linked to original sources
PubMed1991

The ecologic method in the study of environmental health. I. Overview of the method.

This paper summarizes the salient features of the ecologic method, with emphasis on its application in the study of environmental health. Various types of ecologic design are described, with examples. Finally, the main advantages and disadvantages are indicated. A companion paper discusses the methodology of ecologic designs in more detail and describes a census of data sets with potential suitability for the ecologic study of water quality and human health.

Data Interpretation, Statistical↗
PubMed1991

The ecologic method in the study of environmental health. II. Methodologic issues and feasibility.

This paper reviews some methodological aspects of ecologic studies of human health, with emphasis on investigations of environmental quality. A recent census of Canadian and U.S. data sets potentially suitable for this type of study is summarized. It is concluded that despite the considerable utility of the ecologic design for this purpose, substantial practical difficulties are common in their implementation. Particular problems are the relative scarcity of relevant environmental data and complications associated with rendering them compatible with health data.

Canada↗
PubMed1991

A comparison of several point estimators of the odds ratio in a single 2 x 2 contingency table.

The relative performance of the unconditioned maximum likelihood estimators (UMLEs), conditional MLEs (CMLEs), and Jewell-type estimators of the odds ratio (OR) and its logarithm were investigated in sets of single 2 x 2 contingency tables. The tables were generated by complete enumeration of all possible cell frequencies consistent with a single fixed margin. The bias, mean squared error (MSE), and average absolute error (AAE) were computed for all estimators using the individual table probabilities as weights. The results showed that, for the OR, Jewell's estimator usually had smaller bias, MSE, and AAE than either of the MLEs. While the differences were often slight for MSE and AAE, for bias it was sometimes substantial. For the log(OR), the UMLE usually had the lowest bias, and its MSE and AAE were only slightly greater than those for the other estimators. Overall, we recommend estimation on the log scale using the UMLE. If OR is to be estimated, Jewell's method had strong merit, although it is nonsymmetric with respect to the table orientation. In view of this, the UMLE may again be favoured in some situations.

Bias↗
PubMed1991

The estimation of sensitivity and specificity in colorectal cancer screening methods.

The sensitivity and specificity of three screening tests for colorectal cancer were evaluated using latent class analysis. This type of analysis is useful in situations where screening tests are performed on each person and follow-up diagnostic test results are not available for individuals with negative test results. Traditional methods of evaluation, which assume these individuals to be disease free, may be biased under these circumstances. In addition to providing the parameter estimates, the latent class technique gives standard errors and permits significance tests for differences in sensitivity and specificity. It was found that the radial immunodiffusion technique was significantly more sensitive (p less than 0.001) than either the rehydrated or nonrehydrated Hemoccult II tests for detecting occult blood in patients with cancer or adenoma. While comparable to the rehydrated Hemoccult II test in terms of specificity, the radial immunodiffusion technique was significantly less specific (p less than 0.01) than the nonrehydrated Hemoccult test. Similar results were observed when restricting the analysis to cancer only.

Canada↗
PubMed1990

A comparison of multivariable mathematical methods for predicting survival--I. Introduction, rationale, and general strategy.

This paper and the two following papers (Parts I-III) report an investigation of performance variability for four multivariable methods: discriminant function analysis, and linear, logistic, and Cox regression. Each method was examined for its performance in using the same independent variables to develop predictive models for survival of a large cohort of patients with lung cancer. The cogent biologic attributes of the patients had previously been divided into five ordinal stages having a strong prognostic gradient. With stratified random sampling, we prepared seven "generating" sets of data in which the five biologic stages were arranged in proportional, uniform, symmetrical unimodal, decreasing exponential, increasing exponential, U-shaped, or bi-modal distributions. Each of the multivariable methods was applied to each of the seven generating distributions, and the results were tested in a separate "challenge" set, which had not been included in any of the generating sets. The research was intended not merely to compare the performance of the multivariable methods, but also to see how their performance would be affected by different statistical distributions of the same cogent biologic attributes. The results, which are presented in the second and third papers, were compared for selection of independent variables and coefficients, and for accuracy in fitting the generating sets and the challenge set.

Cohort Studies↗
PubMed1990

A comparison of multivariable mathematical methods for predicting survival--III. Accuracy of predictions in generating and challenge sets.

This paper concludes a study of "performance variability" when four methods of multivariable analysis--multiple linear regression, discriminant function analysis, multiple logistic regression, and two arrangements of Cox's proportional hazards regression--were applied to the same stratified random samples of "generating sets" containing seven different statistical distributions of cogent biologic attributes in a composite staging system for a large cohort of patients with lung cancer. Each model developed from the generating sets was also applied for predictions in a previously sequestered "challenge set". Across the different generating sets, the multivariable methods showed good agreement with one another in the stepwise choice of first two powerful predictor variables, but not in the sequence of subsequent choices or in the standardized coefficients assigned to the same collection of "forced" variables. In concordance of predictions for individual patients in the generating sets, the overall proportions of disagreement for pairs of methods ranged from 0 to 28%, and kappa values ranged from 0.49 to 1.00. The accuracy of individual predictions showed relatively similar results when the different methods were applied to the same generating set. Across the generating sets, the different methods showed similar total results but substantial variations in predictions for alive and dead patients. When the models from the generating sets were applied for predictions in the challenge set, the results showed an analogous pattern: similar accuracy within models for overall and live/dead predictions, but substantial variations in live/dead predictions across models derived from different generating sources. The results showed that the multivariable methods often had good agreement with one another in predictions for groups but not for individual persons; and that no single method was superior to the others or to the composite staging system. We conclude that multivariable analytic methods may be most effective and consistent if used to find the few most powerful predictor variables, omitting the many other variables that may be "statistically significant" but less cogent. The powerful predictors may sometimes be best constructed, before the analysis begins, as composite variables containing appropriate unions or ordinal arrangements of elemental candidate variables.

Cohort Studies↗
PubMed1990

The association of cutaneous malignant melanoma with the use of sunbeds and sunlamps.

Data are presented from a large case-control study (583 cases, 608 controls) to estimate the association of melanoma with the use of sunbeds and sunlamps. Odds ratios of 1.88 and 1.45 were found for ever having used a sunbed or sunlamp in males and females, respectively, which was statistically significant in males and of borderline significance in females. These effects persisted when adjustments were made for age and a variety of potential confounders. The effect was slightly stronger for lentigo maligna and for lesions of the face, head, neck, and arms. The risk was greater and significant for both sexes for domestic use of sunbeds/sunlamps, and increased with duration and amount of use. A comparison of 43 cases interviewed before a diagnosis of melanoma had been made with the other 540 cases suggests that recall bias was not responsible for the association. The authors conclude that use of artificial tanning devices appears to be a risk factor for melanoma.

Adult↗
PubMed1989

Simultaneous interval estimates of the odds ratio in studies with two or more comparisons.

More than one odds ratio estimate will often arise from a single epidemiologic study. Examples of designs where this may occur include those where there is more than one case or control group, and investigations of several risk factors as part of the same study. Various methods for presenting multiple interval estimates are discussed, including: the naive method, the Bonferroni method, the Dunn method, the Scheffé method, and the Dunnett method. For rectangular regions the Dunnett method gives a region with the most appropriate confidence level, but this region contains a different set of odds ratio estimates than are implied by the usual significance tests. A confidence ellipse circumscribed by the Scheffé limits gives the best agreement with the significance tests. Each of these methods is illustrated with a numerical example.

Epidemiologic Methods↗
PubMed1989

The feasibility of matching and quota sampling in epidemiologic studies.

Matching is often used to eliminate the effects of potential confounders. The practical execution of a matched design will usually involve additional sampling effort to find appropriate quotas of persons who satisfy the matching criteria. If this effort is substantial, it may render the matched design less attractive than using unmatched samples with covariance adjustment in the analysis. This paper presents numerical results on the sampling effort needed to complete various matched designs: The effort is measured by the number of candidates (N) that must be sampled to identify the matches for analysis. Feasibility of matched sampling is described by the ratio of the expected value of N to its theoretical minimum, and by its coefficient of variation. Feasibility must be weighed against the resulting quality of matches and the possibility of residual confounding. The results show that designs with smaller matching quotas in larger numbers of categories are more difficult to execute. However, the relative variation in N is mainly a function of the category quotas and is only weakly dependent on the number of categories. Recommendations are made regarding the number of categories and the quota sizes that lead to acceptable matched designs. The results are applicable to both observational and experimental studies in which matching or other quota sampling is used.

Clinical Trials as Topic↗
PubMed1989

The clinician's ability to evaluate the strength of healing fractures from plain radiographs.

The present study was designed to analyze the usefulness of plain radiographs in evaluating bone healing. Rabbit tibiae were osteotomized, externally fixed, and allowed to heal for 3-8 weeks. Bones were harvested, x-rayed, and tested to failure in a dynamic torsion tester. AP and lateral radiographs of 10 rabbit tibia pairs and 10 individual rabbit tibiae were selected randomly for use in a questionnaire, given to 93 physicians who routinely assess fracture healing to evaluate clinicians' ability to assess bone strength. The results indicated that clinicians can differentiate the relative strength of bones by comparing two sets of radiographs. However, the strength determination from a single set of radiographs of a fracture is unreliable, the tendency being to evaluate the fracture to be weaker than it actually is.

Animals↗
PubMed1989

The Ontario cohort study of running-related injuries.

A cohort of 1680 runners was enrolled through two community road race events and monitored during a 12-month follow-up period for the occurrence of musculoskeletal injuries. Forty-eight percent of the runners experienced at least one injury, and 54% of these injuries were new; the remainder were recurrences of previous injuries. The risk of injury was associated with increased running mileage but was relatively unassociated with other aspects of training, such as usual pace, usual running surface, hill running, or intense training. Injury rates were equal for all age-sex groups and were independent of years of running experience. Runners injured in the previous year had approximately a 50% higher risk for a new injury during follow-up.

Adolescent↗
PubMed1988

Predicting problems in school performance from preschool health, developmental and behavioural assessments.

To determine the accuracy of various predictors of school problems, we conducted a 3-year prospective study of 1999 children who began school in the Niagara region of Ontario in 1980. During the year before school entry the parents gave a health, developmental and behavioural history during an interview with a community health nurse, and the children underwent vision and hearing screening tests and the Denver Developmental Screening Test (DDST). At the end of the 1980-81 school year the kindergarten teachers rated the children's learning problems. At the end of the 1982-83 school year the presence of school problems was ascertained, and the predictive accuracy of items from the preschool history and examination and of the kindergarten teachers' ratings was calculated. The health, developmental and behavioural history with or without the DDST was found to predict later school problems with acceptable accuracy. The kindergarten teachers' ratings gave slightly more accurate predictions. We conclude that in communities where prompt diagnostic evaluation and effective therapeutic or preventive help can be provided to children identified as being at high risk, health professionals may play a useful role in screening for future school problems.

Age Factors↗
PubMed1988

The effect of non-differential exposure misclassification on estimates of the attributable and prevented fraction.

This paper considers the effect of non-differential exposure misclassification on the population attributable fraction and the population prevented fraction as a function of the sensitivity and specificity of the exposure classification, the true relative risk, and the true prevalence of the exposure. Given a certain set of sensitivity, specificity, and prevalence of the exposure, the apparent attributable fraction is a constant proportion of the true attributable fraction regardless of the true relative risk. This observation does not hold for the apparent prevented fraction and the apparent relative risk, both of which vary with the true relative risk. For both the attributable and the prevented fraction, the sensitivity of the exposure classification has a greater influence on the magnitude of the bias than the specificity; also, the higher the prevalence of the exposure, the larger is the bias caused by the imperfect exposure classification.

Disease↗