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Biomedical subjects

S D Walter

Publications and source records attributed to S D Walter.

At least 163 records · Page 9Linked to original sources
PubMed1976

The estimation and interpretation of attributable risk in health research.

Various measures of attributable risk are discussed together with a rationale for their use as an alternative to relative risk in health research. Methods of estimation are presented for use with three important kinds of epidemiological study design with one dichotomous risk factor for a dichotomous disease outcome; the study designs are then compared with respect to efficiency. Procedures to analyse confounded, polytomous and interacting risk factors are proposed and it shown that there is a simple relationship between two distinct estimators previously suggested for use with deleterious and beneficial (or preventive) factors. Finally the relevance of attributable risk to an assessment of the potential effects of risk factor modification is discussed in the preventive medicine framework.

Epidemiologic Methods↗
PubMed1976

A generalization of a matrix occupancy problem.

An extension of the so-called committee problem is discussed in terms of the occupancy of a certain matrix whose row totals are taken as fixed, and whose cell entries are constrained to lie between zero and defined maxima. A statistic is constructed as a function of the column totals and used to test the significance of a column effect, irrespective of row differences. Two special cases when the cell maxima are constant, first within rows and second within columns, yield a covariance matrix for the column totals whose inverse may be found explicitly, and this in turn leads to a relatively simple expression for the test statistic. Applications of this generalized problem to certain health research situations are discussed, and the adequacy of a X2 approximation to the statistic's null distribution is mentioned.

Biometry↗
PubMed1975

A test for seasonality of events with a variable population at risk.

A statistical significance test to detect seasonality of epidemiological events is described. The method is similar to that of Edwards, but makes it possible to allow for an arbitrary pattern of variation in the population risk, and also for the unequal lengths of time sectors of a cycyle of seasons (e.g., months of a year). From the test it is possible to estimate the amplitude of seasonal variation and the time at which the maximum occurs in a postulated simple harmonic fluctuation; the adequacy of the description of the data by a curve of this kind may be evaluated using a goodness-of-fit test. A numerical example of the calculations is given using some anencephalus data, and the results are compared with those of alternative tests.

Anencephaly↗
PubMedDate not supplied

Methodological issues in the development of the Canadian Cancer Incidence Atlas.

The Canadian Cancer Incidence Atlas is among recent national atlases using incidence rather than mortality data. Methods used to assess the significance and spatial correlation of the age-standardized rates (ASIRs) for the 290 census divisions are described. The expected number of cases by area was used to determine cancer sites with sufficient cases to be mapped. ASIR significance was assessed using a simulation based on a Poisson distribution. The consistency of the observed case distributions with the Poisson distribution was examined. The bootstrap confidence interval (CI) for the ASIR developed by Swift was used in the atlas. Spatial correlation was assessed with Moran's I/I(max) and the significance determined by a simulation in order to allow for area population variation. Data quality indicators typically used for cancer registries were presented, supplemented by a registry questionnaire.

Atlases as Topic↗
PubMedDate not supplied

Small sample estimation of log odds ratios from logistic regression and fourfold tables.

Schaefer has proposed a method to correct the maximum likelihood logistic regression coefficients for bias in small samples. We show here that this reduces, in the special case of a single dichotomous regression variable, to an earlier result of Haldane, for the estimation of a single log odds ratio. This paper reviews various estimators for the log odds ratio in a fourfold table, and compares the properties of two by complete enumeration in sets of tables with small sample sizes. The first of these, also due to Haldane, is based on the addition of 1/2 to each cell of the table; the second is where 1/2 is added to all cells of the table only if a zero frequency arises. We evaluate the bias and mean squared error of both of these estimators in sets of tables with various sample sizes and odds ratios. Haldane's estimator usually has lower bias and MSE, and so we do not in general recommend the practice of adding 1/2 only as necessary. Exceptions might occur if one has good a priori estimates of the outcome probabilities in the two samples under comparison.

Regression Analysis↗
PubMedDate not supplied

Cause-deleted proportional mortality analysis and the healthy worker effect.

This paper evaluates the relationships between various indices of proportional mortality, used in occupational and other settings where denominator data are unavailable. In particular, we examine the necessary conditions for validity of the proportional mortality ratio (PMR) and mortality odds ratio (MOR) as estimates of the standardized mortality ratio (SMR), in general and for the situation where one considers only a subset of deaths, with certain causes of death deleted from the data. We may exclude data in this way to avoid the inflationary effect on the PMRs for some causes of death (e.g. cancer) that results from a 'healthy worker effect' for other causes of death (e.g. heart disease). We find that cause-deleted PMRs and MORs may be unbiased estimates of their corresponding SMR, and these indices appear most useful for specific hazards of employment or relatively infrequent causes of death.

Biometry↗
PubMedDate not supplied

The effect of mode of agitation and type of plastic bag on storage characteristics and in vivo kinetics of platelet concentrates.

We studied the characteristics of platelet concentrates stored for 5 days at 22 degrees C. Platelets were prepared in three plastic bags (PL 732, PL 1240, and CLX) and stored on one of four platelet agitators, 1- or 6-rpm elliptical and 2- or 6-rpm circular rotators. A total of 76 studies were divided among 12 groups, each group being composed of a different storage bag-rotator combination. In vivo recovery and survival were calculated using Indium-111 oxine-labeled platelets injected into autologous volunteers. Platelet recovery was assessed at 2 hours postinjection or as the y-intercept of the multiple-hit model survival curve. Survival was calculated using linear, exponential, and multiple-hit computer models. Linear T 1/2 also was calculated as an index of platelet survival. At 5 days, the pH of all concentrates was above pH 7.0 and platelet counts were above 5.5 X 10(10) per bag except for the PL 732 with the 6-rpm elliptical rotator, which was 4.6 X 10(10) per bag. This combination also showed a significantly higher poststorage lactic dehydrogenase (LDH) discharge compared to the mean of the other 11 groups (23.6 +/- 5.4% vs. 10.4 +/- 3.0%, p less than 0.05); however, the beta-thromboglobulin (beta-TG) release was not statistically different.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Donors↗
PubMedDate not supplied

Age-specific risk factors for lead absorption in children.

The relationship of blood lead levels to environmental and individual characteristics is analyzed in a large sample of children residing near a lead smelting complex, with particular emphasis on the identification of age-related risk factors. Exceptional variation in both blood leads and its determinants within the study region facilitated the simultaneous detection of several significant risk factors for each year of age from 1 to 9 yr. The strongest predictor of blood lead at all ages was air lead, but the secondary risk factors were age dependent. Household dustiness was significantly related to blood lead in young children, especially those under 2 yr of age; soil lead may be an important source of ingested lead for children between 2 and 7 yr. Other significant effects included that of pica at about 2 yr of age, a slight effect of the occupational category of the fathers of 5- to 8-yr-old children, and a tendency for 8- and 9-yr-old boys to have higher blood leads than girls of the same age. Lead concentration in household paint was not a significant risk factor. These results suggest that a multifactorial approach to the prevention of excessive lead absorption by children is required.

Age Factors↗
PubMedDate not supplied

The aetiology of sport injuries. A review of methodologies.

Although participation in many sporting activities has increased dramatically in recent years, the study of injuries sustained during training or participation is still in its infancy. The most commonly used strategy is to describe the characteristics of a suitable case-series. This approach is relatively easy to implement, can be used to estimate the total morbidity load in a population, and can identify the relative frequency of various types of injury. However, the case series method cannot validly identify risk factors for injury or athletes at high risk; similarly, it cannot be used to estimate the absolute level of risk associated with sports participation. Finally, the population from which the injuries arose is often difficult to identify, and the series may not be representative of all injuries occurring in that population, and this may produce quite misleading results. In contrast, a variety of epidemiological designs may be employed to address questions of aetiology and to identify high risk groups of athletes. With careful attention to the underlying population denominators, one may estimate the relative or absolute risk of injury for athletes with given risk characteristics, defined by type and intensity of their participation in sports or by their individual physiology. This is achieved by inclusion of suitable control subjects in the epidemiological sample; these controls may be uninjured athletes or random samples of the general population. The comparison of injured and uninjured groups permits valid inferences to be drawn concerning risk factors, avoiding the many potential biases which affect inferences drawn from injured athletes only.

Adult↗
PubMedDate not supplied

An analysis of the geographic variation in cancer incidence and its determinants in Ontario.

Cancer incidence data for the Ontario Public Health Units in 1980-91 were examined to investigate regional patterns and the existence of outlier values after adjustment for known risk factors. Candidate risk factors were derived from the Ontario Health Survey and the census. Weighted regressions were fit to the data, and the spatial pattern of the residuals was analyzed. The number of outlier data points with significant elevations or reductions in risk was close to that expected by chance. They were dispersed geographically, and occurred in a variety of cancer types. We conclude that, in general, most of the geographic variation in cancer risk can be associated with variation in known risk factors, and that there appear to be no broad regional effects remaining after adjustment for these factors. A few cancer sites provide limited evidence of regional effects that may warrant further investigation.

Adolescent↗