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PubMed2006

Sampling and mapping a soil erosion cover factor by integrating stratification, model updating and cokriging with images.

Cost-efficient sample designs for collection of ground data and accurate mapping of variables are required to monitor natural resources and environmental and ecological systems. In this study, a sample design and mapping method was developed by integrating stratification, model updating, and cokriging with Landsat Thematic Mapper (TM) imagery. This method is based on the spatial autocorrelation of variables and the spatial cross-correlation among them. It can lead to sample designs with variable grid spacing, where sampling distances between plots vary depending on spatial variability of the variables from location to location. This has potential cost-efficiencies in terms of sample design and mapping. This method is also applicable for mapping in the case in which no ground data can be collected in some parts of a study area because of the high cost. The method was validated in a case study in which a ground and vegetation cover factor was sampled and mapped for monitoring soil erosion. The results showed that when the sample obtained with three strata using the developed method was used for sampling and mapping the cover factor, the sampling cost was greatly decreased, although the error of the map was slightly increased compared to that without stratification; that is, the sample cost-efficiency quantified by the product of cost and error was greatly increased. The increase of cost-efficiency was more obvious when the cover factor values of the plots within the no-significant-change stratum were updated by a model developed using the previous observations instead of remeasuring them in the field.

Cost-Benefit Analysis↗
PubMed1998

Prevalence and pattern of smoking in Pakistan.

OBJECTIVE: To determine the prevalence and pattern of smoking amongst Pakistanis. SAMPLING DESIGN: A two stage stratified sample design was adopted for the survey, Primary Sampling Units (PSU's) and Secondary Sampling Units (SSU's). SETTING: Urban and Rural, Pakistan 1990-94. SUBJECTS: Stratified systematic sample of 9441 males and females aged 15 years and above. MAIN OUTCOME MEASURES: Smokers were defined as, subjects who were currently smoking and who had smoked > or = 100 cigarettes/beedis or chillum/huqqa in their life time. RESULTS: In Pakistan 21.6% (36% males and 9% females) of 9441 subjects were smokers. In urban areas it was 20.7% and in rural 22.0%, males were predominant in both urban and rural areas. Proportion of smokers who used cigarette/beedi were significantly higher in males (60%) while chillum/huqqa were more in females (62%). Prevalence increased with age upto 64 years, after which it declined but in urban females it continued to rise with age. Among both males and females; illiterate, married individuals with poor general health were more likely to smoke. These factors were 2 to 3 times more in males and 2 to 5 times more in females who were more likely to be smokers than those who were literate, single individuals with good general health. CONCLUSION: Smoking was more prevalent in illiterate, married persons and those with poor general health.

Adolescent↗
PubMed2000

The Collaborative Longitudinal Personality Disorders Study: development, aims, design, and sample characteristics.

This paper describes the aims, background, design, and methods used in a collaborative longitudinal study of Axis II personality disorders (PDs). This study examines the putative stability of selected PD diagnoses and criteria, what factors affect their course, and whether their stability and course distinguishes them from a representative Axis I disorder. This article also describes the acquisition and demographics of the sample on whom the study is being done. A prospective, repeated measures investigation of the stability of PDs is now underway at multiple clinical settings in four collaborating urban sites in Boston. New Haven, New York, and Providence. Diagnostic assignments are based on semistructured interview assessments (by clinically trained raters) and confirmed by at least one additional contrasting diagnostic method. The sample consists of 668 treatment seeking and reliably diagnosed adults recruited from a broad range of clinical sites. By design, patients in the sample met standards for one of five diagnostic subgroups: (a) schizotypal (N = 86); (b) borderline (N = 175); (c) avoidant (N = 157); (d) obsessive-compulsive (N = 153) personality disorders or a control group having (e) major depressive disorder without personality disorder (N = 97).

Adolescent↗
PubMed2007

Curvature-adjusted optimal design of sampling times for the inference of pharmacokinetic compartment models.

In pharmacokinetics, compartment models are often used to describe the time course of blood concentration after the administration of a drug. In this article, we propose an optimal design criterion for precise estimation of parameters included in the compartment model and illustrate the non-sequential design of sampling times of blood drug concentration data in individual pharmacokinetics. The proposed optimal design criterion minimizes the determinant of the mean-squared error matrix of the parameter estimator that is quadratically approximated by the curvature array. Therefore, the proposed criterion considers the intrinsic and parameter-effects nonlinearity underlying the compartment model, and so is applicable in a pharmacokinetic experiment where the sample size of the blood drug concentration data is quite small.

Biological Availability↗
PubMed1990

A health-based approach for sampling shallow soils at hazardous waste sites using the AALsoil contact criterion.

Strategies for sampling shallow soils at hazardous waste sites are employed primarily to evaluate levels and distributions of contamination. The ensuing analyses of potential public health impacts are therefore dependent on the sampling design rather than having the sampling design based on the data needs for evaluating potential public health impacts of shallow soil contamination. We define a specific objective that guides the sampling of shallow soils. The sampling results can thereby be directly employed to evaluate potential public health impacts from direct contact exposures to shallow soil contamination.

Animals↗
PubMed2001

Iohexol as a marker of glomerular filtration rate in patients with diabetes: comparison of multiple and simplified sampling protocols.

AIMS: To report on the reproducibility of iohexol glomerular filtration rate (GFR) estimation, to compare the plasma clearance of iohexol with that of[51Cr]EDTA and to evaluate the reliability of reduced sampling schedules in estimating GFR in Type 1 and Type 2 diabetes mellitus. METHODS: Agreement was assessed in 15 Type 1 and 26 Type 2 diabetics with creatinine ranging from 53 to 564 micromol/l. RESULTS: The regression between multiple-sample iohexol and[51Cr]EDTA clearances was 0.999 in Type 1 and 0.987 in Type 2 diabetes (P < 0.0001 for both). A seven-sample design and the three-sample approach by Brøchner-Mortensen were validated by comparison with the full-sample schedule in 87 patients (51 Type 1, 36 Type 2). Full-sample GFR was 80.3 +/- 43.8, seven-sample 79.5 +/- 43.9 (r = 0.990) and three-sample 79.8 +/- 45.2 ml.min-1.1.73 m-2 (r = 0.972). The coefficients of variation of GFR were 2.7 +/- 1.4% and 3.8 +/- 1.9% for the full-sample and the seven-sample approaches, respectively, and significantly higher for the three-sample design (6.9 +/- 3.4%, P = 0.0001). CONCLUSIONS: After iohexol injection, the Brøchner-Mortensen schedule does not provide an accurate estimate of GFR. The seven-sample approach gives acceptable errors and allows a good estimate of GFR throughout a wide range of renal function.

Adult↗
PubMed1995

Study design and sample sizes for a lacI transgenic mouse mutation assay.

Design features that adjust and account for excess variation in a transgenic mouse mutation assay based on a lacI target transgene from E. coli are considered. These features include proper identification of plate, packaging reaction, and animal identifier codes throughout the experimental and analysis phases of the study, "blocking" of exposed and unexposed animals when preparing and plating multiple packaging reactions from the same genomic DNA sample, separating sectored mutant plaques and complete mutant plaques before performing any quantitative analyses, and testing for sources of excess variation attributable to features of the experimental protocol--such as plate-to-plate (within packaging reactions), packaging reaction-to-packaging reaction (within animals), and animal-to-animal (within study). Control and ethylnitrosourea-treated animal data are presented from a fully designed study in the lacI assay. The study design incorporates many of these experimental principles. Statistical methods to identify excess variability are noted, and the designed study data are used to illustrate the types of variability encountered in practice. A standard statistical test for two-sample testing is highlighted, from which recommendations are made for sample size selection in future studies.

Analysis of Variance↗
PubMed2000

Diagnostic strategies for the histological examination of muscle biopsy specimens for the assessment of vasculitis in rheumatoid arthritis.

For the diagnosis of rheumatoid vasculitis (RV), histological examination of a blindly-taken muscle biopsy is advocated. If fibrinoid necrosis (FN) is observed in one or more tissue sections of the biopsy the diagnosis of RV is confirmed. The diagnostic value of such histological investigation depends on the prevalence of FN in biopsies of RV patients, the number of tissue sections that are investigated, and the sampling design with which the tissue sections are obtained from the biopsy. In this paper we determine the mathematical relation between the sensitivity of the RV diagnosis and these three factors for four different models for the FN distribution in RV biopsies. The goodness-of-fit of these models was assessed by analysing 18 829 tissue sections of the biopsies of 56 patients, among which were 24 (otherwise histologically proven) RV patients. It appeared that the sensitivity was moderate: FN was observed in only 14 out of 24 (58 per cent) muscle biopsies of the RV patients. The prevalence of FN in the tissue sections of those 14 biopsies was low, about 17 per cent according to the best fitting model. We proved that the sampling design, maximizing the minimum distance between tissue sections (equidistant sampling) was optimal, and that with such optimal sampling, examination of about 20 tissue sections was sufficient. With practical sampling designs, however, considerably more tissue sections had to be inspected. FN appeared to cluster in the RV biopsies, and a first-order Markov model satisfactorily described the (auto)correlation between adjacent tissue sections in RV biopsies.

Arthritis, Rheumatoid↗
PubMed2002

Risk factors for domestic violence: findings from a South African cross-sectional study.

In 1998 a cross-sectional study of violence against women was undertaken in three provinces of South Africa. The objectives were to measure the prevalence of physical, sexual and emotional abuse of women, to identify risk factors and associated health problems and health service use. A multi-stage sampling design was used with clusters sampled with probability proportional to number of households and households were randomly selected from within clusters. One randomly selected woman aged 18-49 years was interviewed in each selected home. Interviews were held with a total 1306 women, the response rate was 90.3% of eligible women. For the risk factor analysis, multiple logistic regression models were fitted from a large pool of candidate explanatory variables, while allowing for sampling design and interviewer effects. The lifetime prevalence of experiencing physical violence from a current or ex-husband or boyfriend was 24.6%, and 9.5% had been assaulted in the previous year. Domestic violence was significantly positively associated with violence in her childhood, her having no further education, liberal ideas on women's roles, drinking alcohol, having another partner in the year, having a confidant(e), his boy child preference, conflict over his drinking, either partner financially supporting the home, frequent conflict generally, and living outside the Northern Province. No significant associations were found with partners' ages, employment, migrant status, financial disparity, cohabitation, household possessions, urbanisation, marital status, crowding, communication, his having other partners, his education, her attitudes towards violence or her perceptions of cultural norms on women's role. The findings suggest that domestic violence is most strongly related to the status of women in a society and to the normative use of violence in conflict situations or as part of the exercise of power. We conclude by discussing implications for developing theory on causal factors in domestic violence.

Adolescent↗
PubMed1988

Bayesian methods for multiple capture-recapture surveys.

To estimate the total size of a closed population, a multiple capture-recapture sampling design can be used. This sampling design has been used traditionally to estimate the size of wildlife populations and is becoming more widely used to estimate the size of hard-to-count human populations. This paper presents Bayesian methods for obtaining point and interval estimates from data gathered from capture-recapture surveys. A numerical example involving the estimation of the size of a fish population is given to illustrate the methods.

Animals↗
PubMed1991

Cognitive abilities and achievement status of children with myelomeningocele: a contemporary sample.

Designed study as a conceptual replication of Shaffer, Friedrich, Shurtleff, and Wolf (1985). Intelligence, school achievement, and perceptual motor skill data from 73 children with uncomplicated myelomeningocele were examined to determine their deviation from test norms. The respective impact of shunting and functional motor level was also assessed. The sample as a whole functioned below WISC-R and VMI norms. On the WRAT, deviation from the norms was evident only for the Arithmetic subtest. Partial replication of the Shunt X Motor level interaction reported by Shaffer et al. (1985) was obtained. Limitations of small-sample studies are discussed.

Adolescent↗
PubMed2003

Experimental design and sample size determination for testing synergism in drug combination studies based on uniform measures.

In anticancer drug development, the combined use of two drugs is an important strategy to achieve greater therapeutic success. Often combination studies are performed in animal (mostly mice) models before clinical trials are conducted. These experiments on mice are costly, especially with combination studies. However, experimental designs and sample size derivations for the joint action of drugs are not currently available except for a few cases where strong model assumptions are made. For example, Abdelbasit and Plackett proposed an optimal design assuming that the dose-response relationship follows some specified linear models. Tallarida et al. derived a design by fixing the mixture ratio and used a t-test to detect the simple similar action. The issue is that in reality we usually do not have enough information on the joint action of the two compounds before experiment and to understand their joint action is exactly our study goal. In this paper, we first propose a novel non-parametric model that does not impose such strong assumptions on the joint action. We then propose an experimental design for the joint action using uniform measure in this non-parametric model. This design is optimal in the sense that it reduces the variability in modelling synergy while allocating the doses to minimize the number of experimental units and to extract maximum information on the joint action of the compounds. Based on this design, we propose a robust F-test to detect departures from the simple similar action of two compounds and a method to determine sample sizes that are economically feasible. We illustrate the method with a study of the joint action of two new anticancer agents: temozolomide and irinotecan.

Animals↗
PubMed2006

Design-based sampling and quantitation of the respiratory airways.

Design-based quantitation of the nasal cavity, larynx and tracheobronchial conducting airways after exposure to inhaled toxicants requires complete measurement of all respiratory airways or appropriate sampling followed by morphometric measurements. In vivo imaging (MRI or CT) of the nasal cavity, larynx and conducting airways provides anatomical detail of all the airways down to the distal airways. Since inhaled toxicants show predictable deposition patterns in the airways, identification and sampling of conducting airways becomes essential in a precise toxicological evaluation. Lengths, diameters and luminal surface areas can be directly measured on fixed specimens using a stereomicroscope. Estimates of cell numbers, extracellular matrix volumes and vessel/nerve lengths per airway or epithelial basal laminar surface are estimated stereologically. Selected airways are cut into smaller pieces using a "fractionator" for uniform sampling of the airways. Cell numbers are estimated using a "disector." Volumes are estimated using point probes, while length and surface areas are estimated by isotropically oriented sections with plane and line probes; an approach free of assumptions of shape, size or spatial orientation. True biological variance and the average sampling variance of the stereological measurement define the minimal sampling required to achieve precise estimates of the nasal cavity, larynx and conducting airways.

Animals↗
PubMed1999

Characteristics of low-angle x-ray scattering from some biological samples.

Design of medical imaging devices based on the detection of low-angle coherent scattering is a subject of increasing interest. The technique is based on the differences in the distribution of photons coherently scattered from different body tissues. Coherent scattering is also useful in monitoring changes that may occur in a healthy tissue (e.g. carcinoma). In this work, low angle scattering properties of some tissues and tissue-equivalent materials are studied. Special care is given to the possibility of distinguishing between tissues of similar water content (e.g. muscle and blood). For this purpose, a Monte Carlo simulation is updated, introducing molecular form factor data, which include molecular interference effects. This program is used to simulate the angular distribution of scattered photons from two tissue-equivalent materials (lucite and water) and three biological samples (muscle, fat and blood). Simulation results agree well with previously measured angular distributions of scattered photons at 59.54 keV. Scattering from water and lucite is also measured at 8.047 keV. The effects of scattering geometry, sample thickness, incident photon energy and tissue type on the angular distribution of scattered photons are investigated. Results reveal the potential of measuring the full width at half maximum (FWHM) of the scattered photon distribution for tissue characterization. Energies up to 13 keV and sample thickness of 0.3 cm reported maximum differences between investigated samples. These conditions are expected to maximize the potential of using coherent scattering set-ups to monitor changes in biological samples even if their water contents are similar. Present results may act as a guide for the optimization of coherent scattering imaging systems.

Adipose Tissue↗
PubMed1999

National Health Interview Survey: research for the 1995-2004 redesign.

OBJECTIVES: This report presents a major portion of the research carried out for the 1995-2004 redesign of the National Health Interview Survey. The primary focus of the 1995-2004 NHIS redesign research was to explore sample design options to improve the reliability of NHIS statistics for racial, ethnic, economic, and geographic domains. Another objective was to assess the integration of the sample designs of the NCHS surveys of health care providers and NHIS. METHODS: A number of research tasks were carried out by Westat, Inc., to explore the feasibility and cost of various sample design options for the 1995-2004 National Health Interview Survey redesign. This report provides a detailed description of the research that was carried out. RESULTS: The research results presented in this report were the primary basis for assessing the technical soundness and costs for a number of design alternatives. The first option, called the alpha option, was developed under the assumption of a 50-percent data collection budget increase. The second option, called the beta option, was developed under the assumption of no change in the data collection budget. The beta option was the design implemented in 1995. Other important research items described in this report include oversampling methods for minority populations and dual-frame sampling methods. The research results in this report also led to additional research, primarily in the area of survey integration. A more detailed description of the 1995-2004 NHIS design will appear in a forthcoming NCHS Series 2 report titled Design and Estimation for the 1995-2004 National Health Interview Survey.

Adolescent↗
PubMed2005

Performance evaluation of functional medical imaging compression via optimal sampling schedule designs and cluster analysis.

In previous work we have described a technique for the compression of positron emission tomography (PET) image data in the spatial and temporal domains based on optimal sampling schedule designs (OSS) and cluster analysis. It can potentially achieve a high data compression ratio greater than 80:1. However, the number of distinguishable cluster groups in dynamic PET image data is a critical issue for this algorithm that has not been experimentally analyzed on clinical data. In this paper, the problem of experimentally determining the ideal cluster number for the algorithm for PET brain data is addressed.

Algorithms↗
PubMedDate not supplied

Survey sampling issues in primary care research.

Survey research methods are often used to answer questions in primary care medicine. Consequently, investigators must understand and address sampling issues. Probability sampling designs have particular utility for questions that seem most appropriate for survey research. These designs provide each member of the population with a known probability of selection, and if performed properly with an appropriate sample size, they provide a sample with essentially no selection bias. Examples of random and stratified sampling designs are presented. These two probability designs, along with nonprobability sampling techniques (convenience, purposive, quota), are discussed in terms of bias, error, and possible limitations.

Humans↗
PubMed2005

Using the Peters--Belson method to measure health care disparities from complex survey data.

The Peters-Belson (PB) method uses regression to assess wage discrimination and can also be used to analyse disparities for a variety of health care issues, e.g. cancer screening. The PB method estimates the proportion of an overall disparity that is not explained by the covariates in the regression, e.g. education, which may be due to discrimination. This method first fits a regression model with individual-level covariates to the majority/advantaged group and then uses the fitted model to estimate the expected values for minority-group members had they been members of the majority group. The data on disparities in health care available to biomedical researchers differ from data used in legal cases as it is often obtained from large-scale studies or surveys with complex sample designs involving stratified multi-stage cluster sampling. Sample surveys with a large representative sample of various racial/ethnic groups and the extensive collection of important social-demographic variables provide excellent sources of data for assessing disparity for a wide range of health behaviours. We extend the PB method for multiple logistic and linear regressions of simple random samples to weighted data from complex designed survey samples. Because of the weighting and complex sample designs, we show how to apply the Taylor linearization method and delete-one-group jackknife methods to obtain estimates of standard errors for the estimated disparity. Data from the 1998 National Health Interview Survey on racial differences in cancer screening among women is used to illustrate the PB method.

Adult↗