Exposure measurement is at the heart of occupational safety and health practice, and the ability to properly interpret measurement results is critical to worker protection efforts. Statistical ...techniques provide the tools needed to account for measurement variability in order to draw valid inferences about exposures and make defensible decisions in exposure control planning and program management. The concepts underlying the simple hypothesis testing approach using one‐sided and two‐sided confidence intervals are applied in several types of parametric hypothesis tests based on the normal probability distribution. These include Student's t‐test, the two‐sample t‐test, and the paired‐sample t‐test. Measurement data are not always normally or lognormally distributed. Instead, nonparametric or “distribution‐free” techniques are used. The nonparametric equivalent of the two‐sample t‐test is the Mann–Whitney U test, which operates on the ranks of the data rather than the original values. Occupational exposures are often lognormally distributed, i.e. they are skewed to the right.
Adult periodontitis has been described as a chronic infectious process exhibiting sporadic, acute exacerbations which cause quantal, localized losses of dental attachment. Many analytic problems of ...periodontal trials are similar to those of other chronic diseases. However, the episodic, localized, infrequent, and relatively unpredictable behavior of exacerbations, coupled with measurement error difficulties, cause some specific problems. Considerable controversy exists as to the proper selection and treatment of multiple site data from the same patient for group comparisons for epidemiologic or therapeutic evaluative purposes. This paper comments, with varying degrees of emphasis, on several issues pertinent to the analysis of periodontal trials. Considerable attention is given to the ways in which measurement variability may distort analytic results. Statistical treatments of multiple site data for descriptive summaries are distinguished from treatments for formal statistical inference to validate therapeutic effects. Evidence suggesting that sites behave independently is contested. For inferential analyses directed at therapeutic or preventive effects, analytic models based on site independence are deemed unsatisfactory. Methods of summarization that may yield more powerful analyses than all‐site mean scores, while retaining appropriate treatment of inter‐site associations, are suggested. Brief comments and opinions on an assortment of other issues in clinical trial analysis are preferred.
Cover methods quantify vegetative communities in only 2 dimensions. The addition of height measurements to cover data, resulting in canopy volume estimates, provide a more practical level of ...description for shrub communities. We evaluated a technique to estimate canopy volume of shrubs that used a formula 2/3piH (A/2 x B/2) derived from the basic ellipsoid volume formula. Objectives of this study were to determine if there were significant differences among means of repeated observations on sample units: (1) among observers; (2) within observers; and (3) between sample periods when using this technique. At 2 locations in Wyoming, 10 planeleaf willow (Salix planifolia var. planifolia Pursh) plants along each of 5 randomly established transects were sampled during 2 consecutive periods by 4 observers. Differences among observers were significant at both sites (P < 0.05). However, within observer variation between sample periods was not significant (P > 0.05) at either site. Mean canopy volume did not vary significantly (P > 0.05) between sample periods when averaged across observers. Estimated sample sizes ranged between 2 and 31 transects depending on the desired sampling precision and confidence level. The average time per transect among all observers decreased from 13 minutes (SD = 3.7) in sample period 1 to 9 minutes (SD = 1.3) in sample period 2. Using this method, managers can better describe and monitor trends in the structural diversity of shrub communities. This canopy volume technique can be applied with minimal training and is precise, efficient, and repeatable.
Uptake of particles by phagocytosing cells is a process that exhibits variability of its rate. This variability is inherent in the mechanism of particle uptake and in the mechanisms that determine ...the distribution of physiological states within a population of phagocytosing cells. When numbers of particles ingested by cells are determined flow cytometrically an additional measurement variability is superimposed on and interacts with the aforementioned biological variability. In one method of determining population phagocytosis parameters, which involves fitting theoretical equations to experimental time course data on the fractions of cells which have ingested 0, 1, 2, particles, the effects of measurement variability are circumvented, although this usually has the cost of not using all the sample data obtained. However, in a second, simpler, method which is based on determining the time course of the number of particles ingested by an average cell, measurement variability is not circumvented and its effects must be considered. An analysis of the combined effects of biological and measurement variability on the results obtained with the simpler method is presented in this paper. Experimental results for phagocytosis of latex microspheres of uniform size and fluorochrome content by populations of the ciliate Tetrahymena pyriformis show that, for this system, measurement variability is entirely negligible in comparison with biological variability. This conclusion might not apply to other systems, however, and situations which might make measurement variability of some significance are mentioned in the paper. The equations given can be used for the analysis of such situations.
A Bladder Cancer Flow Cytometry Network study has been carried out aimed at identification of the sources of inter‐ and intralaboratory variability. Replicate “cocktail” samples containing a mixture ...of peripheral blood lymphocytes and an aneuploid cell line and samples of peripheral blood lymphocytes serving as a DNA reference standard were distributed to five network laboratories. The samples were stained for DNA using propidium iodide, with each laboratory using its own staining protocol. Sets of these samples were analyzed by flow cytometry to obtain cellular DNA distributions. DNA index and hyperdiploid fraction were calculated for each histogram using an automated technique. Results were evaluated by analysis of variance to identify sources of variability. Three important sources of variation were found that affect flow cytometry in general and‐the transportability of flow cytometry results to routine clinical use in particular. The significant variation among laboratories that is constant across time most probably represents stable differences in instrumentation, instrument set‐up, and laboratory techniques. This variation can be compensated for, if it is known and stable, to develop transportable classification criteria. The second type of variation, termed the interaction component, represents differences among laboratories that are not constant across time. Sources of this variation include inconsistency in sample preparation, staining, and analysis. The elimination of this type of variation is required for meaningful comparison of data within and among laboratories and the creation of interlaboratory databases. The third type of variation represents pure measurement variability and affects the sensitivity of the technique.
This study evaluates changes over time in occupational exposure to n-hexane
by longitudinal repeated measurements analysis of data from the Biological
Monitoring Registry from 1991 to 1998. The main ...sources of variability in
n-hexane exposure among manufacturing workers in Florence province (Italy)
are inspected. The 2,5-hexanedione concentrations in urine of industrial workers are
explained by structural, individual and factory information. Here we analyse the
effectiveness of a 1994 law on workplace conditions based on variability
decomposition of measured 2,5-hexanedione concentrations.
We propose a hierarchical Bayesian model which takes into account the different
levels of aggregation of data. The results show that for leather and shoe factories,
the within-subject and within-factory variance components remain the most important
over the time of study, whereas the between-factory components decreased in
accordance with the expected effect of the new legislation.
Celotno besedilo
Dostopno za:
CEKLJ, DOBA, IZUM, KILJ, NUK, ODKLJ, OILJ, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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