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Biomedical subjects

S Selvin

Publications and source records attributed to S Selvin.

At least 19 recordsLinked to original sources
PubMed1996

Case-control study on maternal residential proximity to high voltage power lines and congenital anomalies in France.

The literature indicates that exposure to electro-magnetic fields (EMF) may result in an increased incidence of cancer and spontaneous abortion. The aim of the present study was to determine whether living closer to high voltage power lines (HVPL) increased the risk of congenital anomalies. We studied residential exposure in any municipality in the Central-East Region of France where there was at least one residence within 500 metres of a HVPL. This was a matched case-control design. The cases consisted of all children with congenital anomalies, identified to the population-based registry in Central-East France between 1988-91. We chose two random controls, matched for birth year and municipality, for each case. For every case and control, we measured the distance from the HVPL to the maternal residence at the time of birth of the child as a surrogate for EMF exposure. Using 100 metres from an HVPL as the cut-point between exposure and non-exposure to electro-magnetic fields produced by HVPL, yielded an odds ratio of 0.95 (95% (confidence interval) CI: 0.45-2.03). Using 50 metres as the cut-point, yielded an OR of 1.25 (95% CI: 0.49-3.22). Among the 11 cases within 100 metres, there were 2 children with chromosomal anomalies, but otherwise there was no pattern in the occurrence of specific anomalies. These data indicate a lack of association between distance to HVPL and the total number of congenital anomalies. This study does not have enough statistical power to determine whether the prevalence of a specific congenital anomaly is significantly increased as a result of living near a HVPL.

Case-Control Studies↗
PubMed1996

Analysing the relationship between maternal weight gain and birthweight: exploration of four statistical issues.

Four statistical issues concerning the analysis of birthweight and maternal weight gain during pregnancy are discussed: (1) Part-whole correlation is described (e.g. the correlation between total maternal weight gain and her infant's birthweight). (2) The choice between a ratio or two separate explanatory variables is explored (e.g. body mass index or using maternal weight and height separately). (3) Two statistical properties (bias and power) when a binary variable replaces a continuous variable are discussed (e.g. consequences of using low birthweight instead of reported birthweight). (4) A model selection procedure is presented to provide a way to select a useful subset of variables from a large number of available explanatory variables to model an outcome variable (e.g. birthweight). These issues are illustrated with a set of 4017 births from Moffitt Hospital at the University of California, San Francisco. Furthermore, these four issues arise in a number of applications of statistical methods to data collected to study the epidemiology of newborn infants.

Bias↗
PubMed1995

Hierarchical cluster analysis for exposure assessment of workers in the Semiconductor Health Study.

The fabrication of integrated circuits in the semiconductor industry involves worker exposures to multiple chemical and physical agents. The potential for a high degree of correlation among exposure variables was of concern in the Semiconductor Health Study. Hierarchical cluster analysis was used to identify groups or "clusters" of correlated variables. Several variations of hierarchical cluster analysis were performed on 14 chemical and physical agents, using exposure data on 882 subjects from the historical cohort of the epidemiological studies. Similarity between agent pairs was determined by calculating two metrics of dissimilarity, and hierarchical trees were constructed using three clustering methods. Among subjects exposed to ethylene-based glycol ethers (EGE), xylene, or n-butyl acetate (nBA), 83% were exposed to EGE and xylene, 86% to EGE and nBA, and 94% to xylene and nBA, suggesting that exposures to EGE, xylene, and nBA were highly correlated. A high correlation was also found for subjects exposed to boron and phosphorus (80%). The trees also revealed cluster groups containing agents associated with work-group exposure categories developed for the epidemiologic analyses.

Cluster Analysis↗
PubMed1995

Maternal weight gain pattern and birth weight.

OBJECTIVES: To determine the relationship between maternal weight gain pattern and birth weight. METHODS: All nonobese, white women delivered at the University of California, San Francisco, between 1980-1990 were eligible for this study. Our study group included 2994 uncomplicated pregnancies with complete data. All recorded prenatal weight gain measurements were used to estimate maternal trimester weight gain, pattern of gain (based on low versus not-low gain at each trimester), and total gain at delivery. Multiple linear regression analysis was used to assess the relationship between these weight gain measurements and fetal birth weight. RESULTS: After adjustment for seven covariates, each kilogram of maternal gain in the first, second, and third trimesters was associated with statistically significant increases in fetal birth weight of 18.0, 32.8, and 17.0 g, respectively. When compared with the pattern of gain that was not low in any trimester, patterns with low gain in the first and second trimesters or in the second and third trimesters were associated with significant decreases in birth weights of 133.0 and 88.5 g, but no important change in birth weight was seen for the group whose gains were low in the first and third trimesters. These findings were not due to differences in total weight gain, which averaged approximately 11 kg in these three pattern groups. CONCLUSION: The results suggest that specific patterns of maternal weight gain, particularly weight gain during the second trimester, are related to fetal birth weight.

Adult↗
PubMed1995

Factors associated with the pattern of maternal weight gain during pregnancy.

OBJECTIVE: To examine the pattern of maternal weight gain using maternal characteristics and pregnancy outcome. METHODS: We used maternal weight data measured prospectively from all deliveries between 1980-1990 at the University of California, San Francisco. Piecewise linear regression was used to estimate the rate of maternal weight gain in each trimester. Bivariate techniques were used to examine associations between maternal weight gain per trimester and maternal characteristics and pregnancy outcomes. We also used multiple regression analysis to examine the relationship between maternal characteristics and trimester weight gain. RESULTS: Weight data for at least one trimester were available for 10,418 women. The average rate of weight gain (kg/week) was lowest during the first trimester (0.169 +/- 0.268, n = 7587), peaked during the second trimester (0.563 +/- 0.236, n = 8000), and slowed slightly in the third trimester (0.518 +/- 0.234, n = 10,052). Maternal height, hypertension, cesarean delivery, and fetal size correlated positively with the rate of gain in each trimester, but pre-pregnancy body size, age, parity, smoking status, race-ethnicity, and diabetes were associated differently with gain, depending on which trimester was examined. The most important maternal predictors of weight gain per trimester were age and Asian race-ethnicity in the first trimester; pre-pregnancy body mass, parity, and height in the second; and hypertension, age, and parity in the third. CONCLUSION: Maternal weight gain per trimester is associated with a number of maternal characteristics and pregnancy outcomes, and these relationships vary according to which trimester is being examined.

Adult↗
PubMed1993

Interfacing U.S. census map files with statistical graphics software: application and use in epidemiology.

In 1990, the United States Bureau of the Census released detailed geographic map files known as TIGER/Line (Topologically Integrated Geographic Encoding and Referencing). The TIGER files, accessible through purchase or federal repository libraries, contain 24 billion characters of data describing various geographic features including coastlines, hydrography, transportation networks, political boundaries, etc. for the entire United States. Many of these physical features are of potential interest in epidemiological case studies. Unfortunately, the TIGER data base only provides raw alphanumeric data; no utility software, graphical or otherwise, is included. Recently, the S statistical software package has been extended to include a map display function. The map function augments S's high-level approach towards statistical analysis and graphical display of data. Coupling this statistical software with the map data base developed for U.S. Census data collection will facilitate epidemiological research. We discuss the technical background necessary to utilize the TIGER data base for mapping with S. Two types of S maps, segment-based and polygon-based, are discussed along with methods to construct them from TIGER data. Polygon-based maps are useful for displaying regional statistical data, such as disease rates or incidence at the census tract level. Segment-based maps are easier to assemble and are appropriate when the data are not regionalized. Census tract data of AIDS incidence in San Francisco and lung cancer case locations relative to petrochemical refinery sites in Contra Costa County are used to illustrate the methods and potential uses of interfacing the TIGER data base with S.

Acquired Immunodeficiency Syndrome↗
PubMed1993

Description and reporting of statistical methods.

A review of the last three volumes of AJIC (1990, 1991, and 1992, numbers 1 through 3) revealed a modest use of statistics in infection control research. Discussion of the statistics most commonly used results in the following recommendations: (1) Researchers who use the t test should report the t test statistic, exact significance probabilities, and a discussion of assumptions and requirements underlying the analysis. (2) A 95% confidence interval for estimated quantities is preferred to p values. (3) The potential impact of nonresponse bias should be included in reports of survey data. (4) An adequate description of statistical methods should be included in all reports of survey data. (5) An adequate description of statistical methods should be included in all research articles.

Confidence Intervals↗
PubMed1993

Interpoint squared distance as a measure of spatial clustering.

The expectation and variance for the mean interpoint squared distance are presented. In order to evaluate these expressions it is necessary to calculate the moments of a bivariate uniform distribution defined over an arbitrary polygon. Expressions for these moments are presented, allowing the mean interpoint squared distance to be used as a measure of spatial clustering. The distribution and power of this test statistic is explored on the unit square, and the spatial distribution of 11 cases of non-Hodgkin's lymphoma is investigated to illustrate an application of the approach.

Cluster Analysis↗
PubMed1993

A dose-response relationship for occupational noise-induced hypertension.

The effect of industrial noise on hypertension prevalence was studied in a group of 1101 female workers in a textile mill in Beijing in 1985. Essentially the entire group had worked in specific workshops in this mill for their full working lives and all had worked for at least five years. The noise levels within the plant were assessed and appear to have been constant since 1954 resulting in a well-defined noise exposure for these workers. A cross-sectional design was used in which blood pressures were determined and questionnaires administered to the workers over a two month period. In addition to demographic information, data was gathered on personal and family history of hypertension, current use of prescription drugs, alcohol, tobacco and salt in the diet. Logistic regression indicated that noise exposure is a significant determinant of hypertension prevalence, but third in order of importance behind family history of hypertension and salt use. Each of the predictor variables exerted an independent influence on risk of hypertension. Cumulative exposure to noise was not an important dose-related variable suggesting that, for those susceptible to the effect, hypertension was manifested within the first five years of exposure.

Adult↗
PubMed1992

Distance and risk measures for the analysis of spatial data: a study of childhood cancers.

Three statistical approaches, used to detect spatial clusters of disease associated with a point source exposure, are applied to childhood cancer data for the city of San Francisco (1973-88). The distributions of incident cases of leukemia (51 cases), brain cancer (35 cases), and lymphatic cancer (37 cases) among individuals less than 21 years of age are described using three measures of clustering: distance on a geopolitical map, distance on a density equalized transformed map, and relative risk. The point source of exposure investigated is a large microwave tower located southwest of the center of the city (Sutro Tower). The three analytic approaches indicate that the patterns of the major childhood cancers are essentially random with respect to the point source. These results and a statistical model for spatial clustering are used to explore distance and risk measures in the analysis of spatial data. Both types of measures of spatial clustering are shown to perform similarly when a specific area of exposure can be defined.

Adolescent↗
PubMed1991

Black-white differences in stage-specific cancer survival: analysis of seven selected sites.

A number of researchers have noted that the black population in the United States generally has less favorable cancer survival than does the white population. It is not clear, however, whether this difference is fully explained by differences in stage of disease at diagnosis. This study uses Surveillance, Epidemiology, and End Results program data from the San Francisco-Oakland (California) Metropolitan Statistical Area for the years 1974-1985 to study survival differences between blacks and whites while controlling for both stage and age at diagnosis. The cancer sites examined are those for which mortality is considered avoidable by early detection and treatment, namely the colon, rectum, bladder, breast, cervix, uterine corpus, and prostate. Stage-specific (local, regional, and remote) survival curves are examined for each cancer site. The site- and stage-specific curves for colon, male rectal, and prostate cancer, supplemented by proportional hazards analyses, indicate no significant stage-specific racial differentials. Stage-specific survival differentials persist for male bladder, female rectal, and breast cancer. The relation between race and stage is more complex for female bladder, cervical, and uterine corpus cancer; for these sites, there is a racial difference at some stages but not all. The consequences for secondary intervention programs are considered for the seven sites in light of these findings.

Black People↗
PubMed1991

A measure of goodness-of-fit for the lognormal model applied to occupational exposures.

The lognormal distribution is often applied to occupational exposures, yet the assumption of lognormality is rarely verified. This lack of rigor in evaluating the appropriateness of the lognormal model has resulted, in part, from the difficulty of applying formal goodness-of-fit tests. When evaluation of model fit has been attempted, occupational hygienists have relied upon probability plotting of exposures rather than upon formal statistical methods. The goal of this work was to develop for the occupational hygienist a simple quantitative evaluation to supplement the probability plot. A measure of goodness-of-fit to the lognormal model based on the ratio of two estimators of the mean of the distribution, the simple or direct estimate of the mean and the maximum likelihood estimate of the mean of a lognormal distribution, is described. This new measure, the ratio metric, is a simple extension of calculations made routinely by many occupational hygienists. Results from using the ratio metric were compared to probability plotting and to two traditional measures of goodness-of-fit, the Lilliefors test and the W test, for two occupational exposure data sets. The results of the ratio and W tests are comparable for a variety of occupational exposure data, but the Lilliefors test is overly conservative and does not detect several cases of gross deviations from lognormality. The ratio metric is an effective alternative to the Lilliefors test and is easier to perform than the W test for the range of data usually encountered by occupational hygienists. Occupational hygienists are encouraged to use the ratio metric in conjunction with the probability plot in evaluating the lognormal assumption.

Air Pollutants, Occupational↗
PubMed1991

Modeling benzene pharmacokinetics across three sets of animal data: parametric sensitivity and risk implications.

Typically, the uncertainty affecting the parameters of physiologically based pharmacokinetic (PBPK) models is ignored because it is not currently practical to adjust their values using classical parameter estimation techniques. This issue of parametric variability in a physiological model of benzene pharmacokinetics is addressed in this paper. Monte Carlo simulations were used to study the effects on the model output arising from variability in its parameters. The output was classified into two categories, depending on whether the output of the model on a particular run was judged to be generally consistent with published experimental data. Statistical techniques were used to examine sensitivity and interaction in the parameter space. The model was evaluated against the data from three different experiments in order to test for the structural adequacy of the model and the consistency of the experimental results. The regions of the parameter space associated with various inhalation and gavage experiments are distinct, and the model as presently structured cannot adequately represent the outcomes of all experiments. Our results suggest that further effort is required to discern between the structural adequacy of the model and the consistency of the experimental results. The impact of our results on the risk assessment process for benzene is also examined.

Animals↗
PubMed1991

A dose response relation for noise induced hypertension.

The effect of industrial noise on the prevalence of hypertension was studied in a group of 1101 female workers in a textile mill in Beijing in 1985. Essentially the entire group had worked in specific workshops in this mill for all their working lives and all had worked for at least five years. The noise levels within the plant were assessed and appear to have been constant since 1954 resulting in well defined noise exposures for these workers. A cross sectional design was used in which blood pressures were determined and questionnaires administered to the workers over a two month period. As well as demographic information, data were gathered on personal and family history of hypertension, current use of prescription drugs, alcohol, tobacco, and salt in the diet. Logistic regression indicated that exposure to noise is a significant determinant of prevalence of hypertension, but third in order of importance behind family history of hypertension and use of salt. Each of the predictor variables exerted an independent influence on risk of hypertension. Cumulative exposure to noise was not an important dose related variable suggesting that, for those susceptible to the effect, hypertension was manifested within the first five years of exposure.

Adult↗
PubMed1990

Variability in protection afforded by half-mask respirators against styrene exposure in the field.

Concentrations of styrene were measured in the breathing zone and inside the facepiece of air-purifying half-mask respirators of 13 workers for three to six 1-hr periods while they were engaged in the production of fiberglass-reinforced products. These data were used to estimate the protection afforded by the respirators. In arriving at these estimates, it was necessary to correct the concentration measured inside the mask for pulmonary retention of styrene by the workers. Workers were classified as sprayers or as other production line workers not directly carrying out spraying operations. An analysis of variance showed no evidence of differences in the level of protection afforded by the respirators between the two job classes. A second analysis of variance showed that protection varied between workers as well as for a single worker during different wearing periods. The geometric standard deviation (GSD) between workers was 1.92, the common within-worker GSD was 2.93, and the total GSD was 3.51. One-half of a population of wearers with similar protection would be expected to experience long-term average workplace protection factors in excess of 44 and one-half below that value. The observation of between-worker and within-worker variability in protection indicates that both sources of variability have to be taken into account in the specification of maximum use concentrations.

Air Pollutants, Occupational↗
PubMed1989

A study of multiple causes of death in California: 1955 and 1980.

Multiple cause of death patterns in California for 1980 were compared to a similar study of deaths conducted in 1955. Primary underlying causes of death changed, mainly reflecting the emergence of respiratory cancer as a major cause of death in 1980. The number of causes reported per death increased from 1955 to 1980, in all age and sex groups. Diseases of the arteries and pneumonia, which are among the most common underlying causes of death, appeared more often on death certificates in both 1955 and 1980 as contributing causes than as underlying the death. Diabetes was studied in detail in the 1955 report, and comparisons were made in 1980 to show increases in the proportions of deaths with this disease and corresponding increases in its prevalence among the living population from the National Health Survey. Multiple cause of death data can provide further information on the prevalence of a fatal disease in a population and its relative role in contributing to mortality, and can also provide new information on diseases that contribute to deaths, which was not previously available in population-based studies of single cause of death.

Adult↗
PubMed1989

The effect of autocorrelation on the estimation of workers' daily exposures.

Daily 8-hr time-weighted average (TWA) measurements may not be independent since production rates, maintenance schedules, work practices, and ventilation can result in trends where consecutive values are correlated (autocorrelation). A sampling program which involves collection of measurements on consecutive days, therefore, can result in biased estimates of the mean and variance of the exposure distribution if a high degree of autocorrelation exists. Three simulated data sets were examined to assess the effects of autocorrelation on the estimation of exposure distributions. Results indicated that about 30% of estimated mean values from a highly-autocorrelated series were outside the 95% confidence interval observed for an uncorrelated series. Three data sets obtained from actual workplaces were found to have relatively little autocorrelation. This suggests that for workplaces such as those analyzed here, a random sampling program may not be necessary, and sequential sampling may produce accurate estimates of the parameters of the exposure distribution.

Air Pollutants, Occupational↗