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PubMedDate not supplied

The impact of work environment, utility, and sampling design on occupational magnetic field exposure summaries.

Most recent epidemiologic studies investigating the potential health effects of occupational magnetic field (MF) exposure have relied on MF measurement data linked to job titles. These measurements are summarized by occupational categories, which represent similar groups of job titles. However, job titles alone explain only a small proportion of exposure variability. A comprehensive MF occupational exposure database was used to (1) develop summary job-specific estimates of magnetic field exposure, (2) evaluate the impact of incorporating work environment data to improve electric and magnetic field exposure assessment, and (3) evaluate the use of random versus nonrandom sampling when estimating mean MF exposure levels by occupational categories. Uniform classification systems were developed for occupational and work environment data. A factorial design was used to summarize and calculate arithmetic means and 95% confidence intervals for occupational MF data, assuming that the total variation in MF exposure resulted from variation in occupation, work environment, utility, worker, and day. Occupation-specific means varied across different work environments, particularly for craft workers. Although within-worker and between-worker variability account for a large proportion (over 50%) of exposure variation, work environment (24%) accounted for more exposure variability than occupation (4.9%) or utility (15%). Some differences were observed when results were compared from surveys that used random and nonrandom sampling; however, these differences were not consistent or systematic. It was concluded that MF exposure assessment should consider work environment in addition to job title to reduce exposure misclassification.

Data Collection↗
PubMed1967

Sample size and optimum sample design in tuberculosis surveys.

Tuberculosis surveys sponsored by the World Health Organization have been carried out in different communities during the last few years. Apart from the main epidemiological findings, these surveys have provided basic statistical data for use in the planning of future investigations. In this paper an attempt is made to determine the sample size desirable in future surveys that include one of the following examinations: tuberculin test, direct microscopy, and X-ray examination.The optimum cluster sizes are found to be 100-150 children under 5 years of age in the tuberculin test, at least 200 eligible persons in the examination for excretors of tubercle bacilli (direct microscopy) and at least 500 eligible persons in the examination for persons with radiological evidence of pulmonary tuberculosis (X-ray). Modifications of the optimum sample size in combined surveys are discussed.

Africa↗
PubMed1999

Periodontal disease estimates in NHANES III: clinical measurement and complex sample design issues.

OBJECTIVE: This paper evaluates the possibility that examiner bias or other factors contributed to an observed decline in pocket depth and gingivitis between the two three-year sequential periods of time (or phases) covered by the Third National Health and Nutrition Examination Survey (NHANES III). METHODS: Prevalences of periodontal conditions were analyzed using data from two sets of repeat oral health examinations by examining dentists of NHANES III sample persons. The first set includes sample persons who were examined twice by the same examining dentist at an interval of one to six weeks. The second set includes sample persons who were assessed on the same day by both an examining dentist and a reference dentist. Other possible sources of error also were evaluated. RESULTS: Overall kappa statistics measuring agreement between or within dental examiners were within the range observed for other periodontal disease surveys. While differences were found among dentists in the prevalence of pocket depth of 4 mm or more, for each group of sample persons assessed by a reference examiner-examining dentist pair, the reference examiner's periodontal measurements closely corresponded to measurements made by the examining dentists. CONCLUSIONS: Differences between dental examiners in prevalences of periodontal conditions may be due in part to the fact that examinees were not randomly assigned to examiners. As a result, the sample persons examined by each dentist may not have been alike in characteristics thought to affect periodontal disease status. These findings suggest that the observed declines in periodontal health status between phases is not due to examiner bias. This unexplained decline may be the result of sampling variation. It is recommended that combined six-year survey results be presented whenever possible.

Bias↗
PubMed2001

The MMT bag for emission source sampling: design and evaluation.

This paper presents the design and evaluation results for a metal-coated multilayer Tedlar (MMT) bag that was developed for the collection of source emissions. The applicability of the MMT bag was evaluated for a number of important greenhouse relevant gases: CO, CO2, CH4, N2O, and total hydrocarbons (THCs). The bag was tested for durability and stability for a range of concentrations of the tested compounds using both laboratory-prepared samples and real source samples. The results show that all tested compounds were more stable when stored in the MMT bag than when stored in a regular Tedlar bag. These compounds can be stored at room temperature for at least 3 months without significant changes in concentration. When properly packed, the MMT bag is durable and may be shipped by air. The MMT bag is lower in cost, lighter in weight, and easier to clean, and it requires less devices during the subsequent laboratory analysis compared with a stainless steel canister, which is often used to collect air and source samples.

Air Pollutants, Occupational↗
PubMed1998

Equivalence test and confidence interval for the difference in proportions for the paired-sample design.

This paper considers a model for the difference of two proportions in a paired or matched design of clinical trials, case-control studies and also sensitivity comparison studies of two laboratory tests. This model includes a parameter indicating both interpatient variability of response probabilities and their correlation. Under the proposed model, we derive a one-sided test for equivalence based upon the efficient score. Equivalence is defined here as not more than 100 delta per cent inferior. McNemar's test for significance is shown to be a special case of the proposed test. Further, a score-based confidence interval for the difference of two proportions is derived. One of the features of these methods is applicability to the 2 x 2 table with off-diagonal zero cells; all the McNemar type tests and confidence intervals published so far cannot apply to such data. A Monte Carlo simulation study shows that the proposed test has empirical significance levels closer to the nominal alpha-level than the other tests recently proposed and further that the proposed confidence interval has better empirical coverage probability than those of the four published methods.

Cannabis↗
PubMed1999

Resorbable bioceramics based on stabilized calcium phosphates. Part I: rational design, sample preparation and material characterization.

It has long been the goal of biomaterials research in the field of orthopedics to develop synthetic structures exhibiting comprehensive bioactivity. In particular, an ideal bone-biomaterial would support the activity of osteoblasts in the development of new bone, while simultaneously being resorbed by osteoclasts as part of the lifelong orderly process of bone remodelling. Such resorbable calcium phosphate-based thin films and bulk ceramics have now been created by the high-temperature processing of a fine precipitate, formed from a colloidal sol and stabilized using an additive such as silicon. The materials have two characteristic features: a phase composition which is a mixture of calcium hydroxyapatite and a silicon stabilized tricalcium phosphate, and a microporous morphology based on inter-connected particles (0.2-1 microm in diameter). X-ray diffraction, infrared spectroscopy, nuclear magnetic resonance spectroscopy, and light scattering experiments indicate that the characteristic phase composition arises during sintering through substitution reactions where silicon enters the calcium phosphate lattice under conditions of high chemical reactivity. The crystallographic features are linked through the glaserite form of the apatite structure.

Biocompatible Materials↗
PubMed1989

Optimizing sampling designs for volume measurements of components of human brain using a stereological method.

Measurements of the cerebral cortical volume used to be very laborious, due to the 3-D complexity of the gyral pattern. Using stereological methods, which allow the quantification of 3-D structures from measurements on 2-D cross-sections, the difficulties have been overcome. In thirty formalin-fixed normal human brains the total volumes were measured by saline displacement. The brains were serially sliced in coronal sections and the fractional areas of the cortex, white matter, central grey structures and ventricles were determined by point-counting. Using Cavaliéri's principle the volumes of these structures were calculated. The average cortical fixed volume was 549 ml (SD +/- 107) corresponding to 54% of the total volume of the hemispheres. The coefficient of error of the cortical volume determinations was 2.6%. The efficiency of the design and the possibilities for optimizing the design are discussed. This point-counting method was preferred to the use of an automatic image analyser, being precise, easy to handle and not interfering with further tissue processing for histological preparation.

Aged↗