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Biomedical subjects

S D Walter

Publications and source records attributed to S D Walter.

At least 73 records · Page 4Linked to original sources
PubMed1995

Sensitivity and specificity of different diagnostic criteria for Behçet's disease according to the latent class approach.

In this study, we estimated the sensitivity and specificity of different criteria (International, Japanese, Mason & Barnes, O'Duffy, James and clinical judgement) for Behçet's Disease using the latent class approach. Thirty-two consecutive Behçet's Disease patients who were initially diagnosed according to clinical judgement, and 56 control patients with other rheumatic diseases, were recruited from the rheumatology out-patient clinic. The patients had their charts reviewed, were interviewed and clinically examined. Various models including different combinations of the six diagnostic criteria were tested. In the model that included all the information gathered, the International (sensitivity = 0.95, specificity = 1.00), Japanese (sensitivity = 1.00, specificity - 0.95) and Mason & Barnes criteria (sensitivity = 1.00, specificity = 0.94) were the most accurate. Clinician judgement also performed very well in identifying a Behçet's Disease case (sensitivity = 1.00, specificity = 0.85). The International, Japanese and Mason & Barnes criteria were the most accurate. The latent class method led to these conclusions without making any initial assumptions about the accuracy of any of the competing criteria (including clinical judgement).

Adult↗
PubMed1994

Calendar effects in the analysis of seasonal data.

A variety of statistical methods exist for analyzing seasonal patterns in epidemiologic data. As a simplification in the calculations, these methods often do not explicitly take into account certain calendar effects, such as the variation in month length, the irregular number of weekend days in each month, and the occurrence of holidays. This paper evaluates the bias caused by failing to recognize these effects. It is found that with the sample sizes commonly encountered in this type of analysis of epidemiologic data, calendar effects have a high probability of producing a spuriously significant seasonal effect, the amplitude of which may be of the same order of magnitude as the true underlying seasonal trend. Therefore, it is recommended that calendar effects be routinely taken into account, and some methods for doing so are proposed.

Data Interpretation, Statistical↗
PubMed1994

A simple test for spatial pattern in regional health data.

The rank adjacency statistic D has been used to summarize the spatial autocorrelation in regional health data. It is defined as the average absolute difference in ranks of the data, taken over all possible pairs of adjacent regions. In this paper, the mean and approximate variance are derived for D, including provision for general weighting of regional pairs. Special results are given for the case of binary weights representing regional adjacencies. Empirical analysis with cancer maps from three countries shows excellent properties of a normal approximation to test the significance of D, eliminating the previous need for simulation. The calculations are further illustrated with data on the spatial pattern of urban/rural residence.

Epidemiologic Methods↗
PubMed1994

Psychosocial effects of PCB contamination and remediation: the case of Smithville, Ontario.

A cross-sectional study design was used to investigate psychosocial effects in the population exposed to PCB contamination and remediation in Smithville, Ontario, Canada. Psychosocial effects were defined as the complex of distress, dysfunction and disability manifested in a wide range of psychological, social and behavioural outcomes as a consequence of actual or perceived contamination. This paper describes the results of logistic regression analyses conducted to investigate the determinants of psychosocial effects of exposure. The data come from an epidemiologic survey of a sample (N = 272) of Smithville households within 3 km of the site, and a sample from a matched comparison community (N = 263). Two types of outcome variables were used as indicators of psychosocial effects. Scores on the general measures of psychosocial health and well-being (the GHQ-20 and the somatic complaints checklist of the SCL-90) for the Smithville sample did not differ from those expected in a 'normal' population, nor did they differ from those in the comparison community. Scores on these outcomes were associated with plausibly linked independent variables, but were not associated with exposure to the PCB site. Results for site-specific outcome measures (concern and health concern) showed that moderate levels of concern reported by Smithville respondents were explained by concern about another local hazardous waste issue and could not be explained by PCB site exposure. We conclude from these results that local community context exerts an important influence on psychosocial effects of environmental contamination. Furthermore, the types of outcome measures employed and the timing of the research in the context of the site history were important factors in our ability to detect psychosocial effects of the PCB contamination and remediation in Smithville.

Adult↗
PubMed1994

The geographic variation of cancer incidence in Ontario.

OBJECTIVES: Our objectives were (1) to describe an analysis of the spatial pattern of cancer incidence in Ontario and (2) to discuss the quality of data in the Ontario Cancer Registry with respect to the accuracy of local cancer rates. METHODS: Cancer incidence rates were calculated for 22 cancer sites in 49 counties of Ontario during 1976 to 1986. Capture-recapture methods were used to estimate completeness of case registration, and completeness of residence information was also assessed. Spatial autocorrelation was used in measuring the geographic pattern of incidence rates. Comparisons were also made between sexes and with earlier data from 1966 to 1975. RESULTS: The quality of the geographic data in the registry appeared good, and corrections for incomplete or inaccurate registration had little impact. About one third of the sex-site combinations showed some evidence of spatial patterning in the cancer rate. Particularly strong regional variation was noted for cancers of the stomach, lung, uterus, and prostate. CONCLUSIONS: The analysis revealed a number of cancers with significant spatial patterning of risk. Further work is needed to relate the cancer data to other information on potential life-style and environmental factors.

Female↗
PubMed1993

Visual and statistical assessment of spatial clustering in mapped data.

Maps have seen increasing use to examine regional variation in health, but there has been little research on the visual perception of spatial patterns in mapped data. Theories of graphical perception suggest that the interpretation of maps is complex relative to other types of graphical material. This paper describes an experiment in which observers assessed a series of maps with respect to their amount of clustering. Maps with various types of spatial pattern were visually distinguishable; comparisons between variants of the same map, however, using different shading and plotting symbols indicated that the method of data representation also had a strong effect on visual perception. There was some evidence for a learning effect in complex maps. The relationship between the visual assessments and a statistical measure of spatial autocorrelation was significant but imperfect.

Cluster Analysis↗
PubMed1993

Assessing spatial patterns in disease rates.

We describe the empirical performance of three indices of spatial autocorrelation (Moran's I, Geary's c and a rank adjacency statistic D) in the analysis of regional cancer incidence data. Heterogeneity in regional population sizes and age structure leads to variable precision in estimated rates; the usual methods for assessing I, c and D, which ignore such heterogeneity, are shown to be liberally biased, especially for c and D. The power of these indices to detect likely disease patterns is estimated by stimulation; the power is quite variable, depending on the exact pattern assumed, although I tends to have the highest power. The null distributions appear quite robust in small samples, even when several regions have no observed case. Preliminary work on the Ontario cancer registry showed generally unimportant effects on the spatial analysis of variation in case registration rates or missing residence data.

Disease Outbreaks↗
PubMed1993

The impact of a public health nurse intervention on influenza vaccine acceptance.

Public health clients in an Ontario community 65 years of age or older were randomly allocated to receive an intervention by a public health nurse during a home visit promoting either influenza immunization or safety measures. There was no statistically significant differences in influenza immunization rates between these two groups (56.1% vs 56.6%). Men were significantly more likely to receive immunization.

Aged↗
PubMed1992

The analysis of regional patterns in health data. I. Distributional considerations.

Regional patterns of health data such as cancer incidence rates are often examined for evidence of environmental effects. In this paper, three measures of spatial clustering are evaluated for use with epidemiologic data. In particular, the effects of variation in regional population structure on the distribution of these measures is considered. It is shown that substantial bias occurs if variation in regional population size is ignored (as has occurred in previous analyses). On the other hand, the methods are robust to small case frequencies and to variation in the regional age distribution. It is recommended that these regional differences be routinely taken into account, which can be done with relatively little additional computation. A companion paper (Walter SD. The analysis of regional patterns in health data. II. The power to detect environmental effects.

Bias↗
PubMed1992

The analysis of regional patterns in health data. II. The power to detect environmental effects.

Three measures of spatial clustering (Moran's I, Geary's c, and a rank adjacency statistic, D) were evaluated for their power to detect regional patterns in health data. The patterns represented various environmental effects: a latitude gradient; residence near a contaminated water supply; disease "hot spots"; relation to socioeconomic status and urbanization; and general spatial autocorrelation. While the methods had high power to detect certain patterns, they were also affected by factors such as the shape of the map, its regional structure, and the spatial distribution of explanatory variables. The power was sometimes low, even for strong geographic trends, particularly for D. Moran's I had the highest power most often. We conclude that use of these methods requires careful specification of the anticipated geographic pattern and awareness of idiosyncratic effects in the study of particular maps.

Bias↗
PubMed1992

Use of host factors to identify people at high risk for cutaneous malignant melanoma .

OBJECTIVE: To determine which host characteristics are risk factors for cutaneous malignant melanoma in order to aim prevention and early detection programs at people at high risk. DESIGN: Case-control study. SETTING: Southern Ontario. SUBJECTS: The 583 case subjects were aged 20 to 69 years and had had malignant melanoma newly diagnosed between Oct. 1, 1984, and Sept. 30, 1986. The 608 control subjects were randomly selected from a list of residents in the study area and were stratum matched for age, sex and municipality. INTERVENTION: Through in-person interviews the interviewer ascertained exposure to putative external risk factors and assessed skin colour and number of nevi on the arm, and the subject reported his or her natural hair colour at age 20 years, eye colour, skin reaction to repeated sun exposure, and freckle and whole-body nevus densities. RESULTS: Although all the host factors mentioned were significantly associated with melanoma risk when considered separately, only hair colour, skin reaction to repeated sun exposure, and self-reported freckle and nevus densities remained significant after backward logistic regression analysis. The odds ratio for melanoma was estimated to be 10.7 in people who had many nevi compared with those who had none (95% confidence interval [CI] 6.6 to 17.4), 4.0 in people who had red hair compared with those who had black hair (95% CI 1.9 to 8.2), 1.9 in people who had many freckles compared with those who had none or few (95% CI 1.3 to 2.8) and 1.4 respectively in people who burned and had a subsequent increase in tan and those who burned and had no increase in tan after repeated sun exposure compared with those who did not burn [corrected]. CONCLUSIONS: Four risk factors for malignant melanoma have been identified. Prospective evaluation of their predictive value should be done. In the meantime, however, these factors should be used to identify people apparently at high risk for malignant melanoma, who can then be targeted for early detection and prevention programs.

Adult↗
PubMed1992

The association of cutaneous malignant melanoma and fluorescent light exposure.

Data are presented from an interview case-control study (583 cases and 608 controls), performed in southern Ontario, Canada, from October 1984 to September 1986, on the association of cutaneous malignant melanoma with exposure to fluorescent light. Males showed a significant trend with cumulative years of occupational exposure and with various indices of exposure to domestic fluorescent light. The risk was more pronounced for lesions on the arms and for superficial spreading melanomas. There was no consistent association in females. These effects were similar when adjusted for other major risk factors for melanoma, including the amount of time spent outdoors occupationally. Comparisons of melanoma cases interviewed before or after diagnosis revealed no evidence of rumination bias. Comparisons of sample data from the same cases and controls by interview and mail questionnaire showed reasonable levels of reliability with no evidence of recall bias. A small sample of subjects was also selected for exposure validation with employers; this revealed very accurate recall of occupational exposure. On the basis of these results, previous epidemiologic studies, and clinical and animal evidence, the authors conclude that fluorescent light exposure remains a potential risk factor for melanoma.

Adult↗
PubMed1992

The natural history of lung cancer estimated from the results of a randomized trial of screening.

The results from a randomized controlled trial of screening for lung cancer in Czechoslovakia have been used to estimate parameters of the natural history, using a model to simulate the disease process and the effects of screening. The results suggest that the period before clinical presentation during which lesions can be detected by screening is very short (seven to eight months). This implies that to detect three-quarters of all lung cancers by screening, two examinations per year are necessary, and that such a program would advance diagnosis by six months if there were complete participation. The results of the trial itself suggest that the benefit, in terms of a reduction in mortality from lung cancer, is likely to be very small.

Adult↗
PubMed1991

Reliability of interviewer and subject assessments of nevus counts in a study of melanoma.

Several types of data are presented concerning the reliability of counting or estimating the density of nevi (moles), a major risk factor for melanoma, using methods typically employed in epidemiologic studies. First, interviewer-derived counts of nevi on the arm produced estimates of inter-observer, inter-subject, temporal and random variability, and their interactions. Second, interviewer-derived arm counts and respondent self-reports of whole body nevus density were compared. Finally, we compared male and female cases and controls with respect to their reported rates of having a relative with a malignant mole. Overall, the intra-observer reliability ranged from 55 to 81%, and was better for observers with more experience. The correlation between the interviewer counts and respondents' self-reported estimates was 0.41. The data on malignant moles in relatives suggest higher reporting rates in male cases and lower reporting in male controls relative to their female counterparts, but there is little difference by sex in the reporting of one's own nevus density.

Adult↗
PubMed1991

Statistical significance and fragility criteria for assessing a difference of two proportions.

This paper compares the traditional methods of statistical inference on the data from biomedical studies with a proposed index of fragility in the results. In general, for any given study there are 8 possible combinations of conclusions regarding statistical significance, quantitative significance and fragility. The 8 possibilities are considered in turn with respect to how studies in each group might be interpreted. Numerical examples show that not all 8 possibilities need be attainable with a given study design, and that the relative likelihood of them occurring can vary widely. It is concluded that the fragility index may be a useful adjunct to conventional statistical inference, with certain intuitive appeal, but that more empirical experience is needed with the fragility method.

Analysis of Variance↗
PubMed1991

Mapping mortality and morbidity patterns: an international comparison.

A set of 49 national, intranational and international health atlases was surveyed to characterize their mapping methodology with respect to the populations covered, the diseases represented, the mapping techniques, and statistical methods. Little consistency was found concerning the choice of data function to be mapped, minimum event frequency requirements, method of age standardization, or map colour systems. Many atlases did not include basic epidemiological information; for instance, approximately half the atlases did not quote population denominators. There was a tendency to emphasize statistical significance over rate values, and to focus on high rather than low risk. Very few atlases included supplemental information on environmental factors. Most adopted a descriptive stance, and attempted no aetiological interpretation. We conclude that inter-atlas comparisons are made very difficult by methodological differences, and that even regional comparisons within atlases should be made cautiously. We propose a set of methodological guidelines for consideration in future atlases.

Epidemiology↗