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At least 199 records · Page 11Linked to original sources
PubMed1996

A comparison of methods for limited-sampling strategy design using data from a phase I trial of the anthrapyrazole DuP-941.

The pharmacokinetics of a drug in individual patients can be estimated using plasma samples collected at a limited number of time points. However, different methods for a limited-sampling strategy (LSS) design exist and the optimal method has not yet been defined. Plasma concentration data were available from 27 of 74 courses in a phase I study (dose range, 5-55 mg m-2) of the novel anthrapyrazole DuP-941. Three approaches to LSS development were compared. Firstly, forward stepwise regression (FSR) was used to derive equations to predict the DuP-941 area under the concentration-time curve (AUC) based on plasma concentrations measured at specified times. LSSs were developed using 14 randomly chosen data sets and were validated using the remaining 13 data sets. Secondly, "all subsets" regression (ASR) was used to develop LSSs. A jack-knife technique was also used to allow model development utilising 26 data sets and validation on the 27th data set. Thirdly, an LSS was developed using optimal sampling theory (OST), and the LSS was used in conjunction with a Bavesian algorithm. Selected sampling times for four-point LSSs were 10, 65, 185 and 485 min (FSR) and 10, 45, 200 and 480 min (OST). Ten candidate LSSs were developed using the ASR approach. ASR- and OST/Bayesian-derived four-point LSSs gave more precise (P < 0.05) estimates of AUC [mean absolute percentage of difference (MAD%) +/- SD: ASR, 6.4 +/- 3.7%; OST/Bayesian, 6.8 +/- 4.6%] than did FSR (MAD% = 15.1 +/- 9.9%). The OST/Bayesian approach is recommended because it allows estimation of all model parameters and is more flexible with regard to sample collection time and design variables.

Adult↗
PubMed1998

Reliability of single sample experimental designs: comfortable effort level.

This study was designed to ascertain the intrasubject variability across multiple recording sessions-most often disregarded in reporting group mean data or unavailable because of single sample experimental designs. Intrasubject variability was assessed within and across several experimental sessions from measures of speaking fundamental frequency, vocal intensity, and reading rate. Three age groups of men and women--young, middle-aged, and elderly--repeated the vowel /a/, read a standard passage, and spoke extemporaneously during each experimental session. Statistical analyses were performed to assess each speaker's variability from his or her own mean, and that which consistently varied for any one speaking sample type, both within or across days. Results indicated that intrasubject variability was minimal, with approximately 4% of the data exhibiting significant variation across experimental sessions.

Adult↗
PubMed1996

Biosafety implications in sample introduction: module design characteristics for discrete-sample systems used in critical whole blood analyte testing environments.

Systems designed for the measurement of pH/blood gases and expanded versions of these systems that include the ability to measure electrolytes or other related quantities, all incorporate one or more sampling modules that are limited in their ability to allow for the safe handling of potentially biohazardous blood samples. Systems that provide for injection of sample from a syringe use more than the required volume for sample testing and have the potential for causing splash-back of the sample if the introduction port is blocked. The probes of aspiration-based systems require manual dexterity by the operator and present the possibility of operator injury with a contaminated probe tip or the possibility of system damage as the probe extends and the operator moves the collection device into place. Some systems have the potential for both types of biohazards. This report describes the implications for system design of the sample collection devices commonly in use, and it offers a design solution that combines the ergonomics of an injection system and the operational advantages of an aspiration system, in combination, addressing the biosafety aspects in a unique fashion.

Blood Gas Analysis↗
PubMed1997

[Estimation methods in a sampling survey].

The objective of this paper is to present a guide for statistical analysis of a sampling survey. Advantages and disadvantages of classical sampling designs are first discussed. A sampling survey was designed to give more precise estimates of parameters which characterize a targeted well-defined population. Simple sampling design often is impossible in large population and rarely is the optimal solution. According to practical and economic constraints, and to available sampling frames, other strategies (stratification, unequal probabilities, several selection steps) may be necessary or more efficient. Fundamental tools to compute estimators and their confidence intervals are presented. The choice of the sampling method determine for each population unit the probability to include it in the final sample. These inclusion probabilities must be known to formulate estimators, ideally unbiased and with low variance. Difficulties arise from calculating the variance, especially for estimators which are not linear functions of the characteristics of interest. In that case, estimation procedures include Taylor linearization. It is always possible to find at least one linear estimator for a total. The total is the key-parameter in sampling theory, most parameters (such as ratios, means, percentages...), being function of unknown totals. Numerical examples are given.

Confidence Intervals↗
PubMed2005

Watershed-based survey designs.

Watershed-based sampling design and assessment tools help serve the multiple goals for water quality monitoring required under the Clean Water Act, including assessment of regional conditions to meet Section 305(b), identification of impaired water bodies or watersheds to meet Section 303(d), and development of empirical relationships between causes or sources of impairment and biological responses. Creation of GIS databases for hydrography, hydrologically corrected digital elevation models, and hydrologic derivatives such as watershed boundaries and upstream-downstream topology of subcatchments would provide a consistent seamless nationwide framework for these designs. The elements of a watershed-based sample framework can be represented either as a continuous infinite set defined by points along a linear stream network, or as a discrete set of watershed polygons. Watershed-based designs can be developed with existing probabilistic survey methods, including the use of unequal probability weighting, stratification, and two-stage frames for sampling. Case studies for monitoring of Atlantic Coastal Plain streams, West Virginia wadeable streams, and coastal Oregon streams illustrate three different approaches for selecting sites for watershed-based survey designs.

Data Collection↗
PubMed1987

Evaluation of efficient designs for observational epidemiologic studies.

Recent research in the design of observational epidemiologic studies has been focused on determining which design is most efficient for controlling a potential confounding factor in the analysis of the disease-exposure relationship. Typically, only two candidate designs have been considered, the matched design and the random sample design. These are merely two of an infinite variety of potential designs in which the distributions of the confounder in the comparison groups and the ratio of group sample sizes are arbitrarily chosen. Only in special cases will either of the standard designs be the most efficient or "optimal" design. We construct optimal designs by minimizing the variance of the desired estimate of effect with respect to the controllable design parameters. Construction of an optimal design depends on unknown parameters, so that in practice only an approximately optimal design, perhaps constructed sequentially, is possible. We evaluate the potential usefulness of optimal designs by identifying circumstances in which an optimal design results in large efficiency gains relative to both the matched and random sample designs. We find that there can be substantial efficiency gains in follow-up studies when both the exposure and confounder are strong risk factors. Practical issues in the implementation of these designs are discussed.

Epidemiologic Methods↗
PubMed2006

The Mayo Clinic cohort study of personality and aging: design and sampling, reliability and validity of instruments, and baseline description.

We established a historical cohort of 7,216 subjects who completed the Minnesota Multiphasic Personality Inventory (MMPI) at the Mayo Clinic from 1962 through 1965 for research (not clinical indication), and who resided within a 120-mile radius centered in Rochester, Minnesota. We describe here the overall cohort design and sampling, we report results concerning reliability and validity, and we describe age and sex patterns at baseline for four MMPI scores of primary interest (depression, anxiety, social introversion, and negativity). Subjects excluded from the cohort because of missing data had MMPI scores similar to subjects included (after appropriate rescaling). A cut-off specific for age and sex at the 75th percentile of the distribution of raw scores was valid compared with the traditional clinical cut-off (T scores plus one standard deviation). Baseline scores for all four scales were higher in women than in men at all ages (all p < 0.0001). Depression and social introversion scores showed an increasing trend with age in both sexes (Spearman rank correlation, rho = 0.05 and 0.08, respectively, p < 0.0001 for both). Baseline scores on the anxiety scale showed a decreasing trend with age in both sexes (rho = -0.06, p < 0.0001). Negativity scores remained relatively stable with age in both sexes (rho = 0.03, p = 0.01). We found a high correlation between the anxiety score and the negativity score (rho = 0.90, p < 0.0001) even after the exclusion of overlapping items (rho = 0.68, p < 0.0001). This newly established historical cohort study provides opportunities to test hypotheses regarding the link between personality and aging, aging-related diseases, and overall mortality.

Adult↗
PubMed2004

Design and sampling methodology for a large study of preschool children's aggregate exposures to persistent organic pollutants in their everyday environments.

Young children, because of their immaturity and their rapid development compared to adults, are considered to be more susceptible to the health effects of environmental pollutants. They are also more likely to be exposed to these pollutants, because of their continual exploration of their environments with all their senses. Although there has been increased emphasis in recent years on exposure research aimed at this specific susceptible population, there are still large gaps in the available data, especially in the area of chronic, low-level exposures of children in their home and school environments. A research program on preschool children's exposures was established in 1996 at the USEPA National Exposure Research Laboratory. The emphasis of this program is on children's aggregate exposures to common contaminants in their everyday environments, from multiple media, through all routes of exposure. The current research project, "Children's Total Exposure to Persistent Pesticides and Other Persistent Organic Pollutants," (CTEPP), is a pilot-scale study of the exposures of 257 children, ages 1(1/2)-5 years, and their primary adult caregivers to contaminants in their everyday surroundings. The contaminants of interest include several pesticides, phenols, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and phthalate esters. Field recruitment and data collection began in February 2000 in North Carolina and were completed in November 2001 in Ohio. This paper describes the design strategy, survey sampling, recruiting, and field methods for the CTEPP study.

Child Day Care Centers↗
PubMed1997

Duration of general practice consultations: association with patient occupational and educational status.

Past studies have demonstrated that the majority of health care visits are made to general practitioners, and that socio-economically disadvantaged individuals are significantly more likely to use such services. Relatively little is known, however, about the quality of general practice care provided to patients of different socio-economic status. The specific aims of the study were to determine whether an association existed between consultation duration and patient educational and occupational status, and if an association was evident, to determine the extent of association after taking into account a range of identified confounding variables and the effect of a clustered sample design. Consecutive consultations from a randomly selected sample of general practitioners were audiotaped and their durations measured electronically. Patient education and occupational status were obtained by questionnaire. Information concerning a range of additional patient, practitioner and consultation variables was also assessed in order to identify possible confounders of the association between consultation duration and patient occupational and educational status. No association was evident between consultation duration and level of patient educational qualification. Independent of identified confounding variables and the effect of a clustered sample design, general practitioners spent less time with those patients employed in unskilled occupations. Unskilled patients received 2.1 min or 21% less time per consultation than patients in professional occupations. The odds of patients in unskilled occupations receiving a long consultation (> 10 min) were 26% less than the odds of patients in professional occupations. The finding of an occupational status differential in the duration of general practice consultations suggests that socio-economically disadvantaged patients may not be receiving the health care they require. Further research is required to confirm these findings and to identify whether similar differentials are evident in more specific elements of general practice care.

Adult↗
PubMed2003

Design and sample size estimation in clinical trials with clustered survival times as the primary endpoint.

Many clinical trials involve the collection of data on the time to occurrence of the same type of multiple events within sample units, in which ordering of events is arbitrary and times are usually correlated. To design a clinical trial with this type of clustered survival times as the primary endpoint, estimating the number of subjects (sampling units) required for a given power to detect a specified treatment difference is an important issue. In this paper we derive a sample size formula for clustered survival data via Lee, Wei and Amato's marginal model. It can be easily used to plan a clinical trial in which clustered survival times are of primary interest. Simulation studies demonstrate that the formula works very well. We also discuss and compare cluster survival time design and single survival time design (for example, time to the first event) in different scenarios.

Cluster Analysis↗
PubMed1989

Optimal design of sampling schedules for studying glucose kinetics with tracers.

Minimum size sampling schedules for estimating glucose kinetic parameters from an impulsive (bolus) tracer injection in normal humans and rats are presented. Glucose kinetics are described by a two-compartment linear model, and reference values of the parameters are estimated from a data base with many samples. The optimal sampling schedule (OSS) is determined in each individual by using a D-optimal criterion and consists of four samples. A population optimal sampling schedule (POSS) applicable to all the individuals of a given population is then determined, and its reliability and efficiency in recovering kinetic parameters (e.g., rate constants, plasma clearance rate, and mean residence time) is assessed. The influence of model and measurement error on OSS is discussed. Moreover, the adoption of an enhanced POSS (EPOSS, 8 samples) is shown to improve accuracy and precision of parameter estimates in a predictable manner. Finally some suggestions are given for obtaining more information from turnover studies using a constant infusion of tracer, with or without a priming pulse of tracer.

Animals↗
PubMed2001

Statistical analysis of spatial pattern: a comparison of grid and hierarchical sampling approaches.

Previous studies have combined random-site hierarchical sampling designs with analysis of variance techniques, and grid sampling with spatial autocorrelation analysis. We illustrate that analysis techniques and sampling designs are interchangeable using densities of an infaunal bivalve from a study in Poverty Bay, New Zealand. Hierarchical designs allow the estimation of variances associated with each level, but high-level factors are imprecisely estimated, and they are inefficient for describing spatial pattern. Grid designs are efficient for describing spatial pattern, and are amenable to conventional analysis. Our example deals with a continuous spatial habitat, but our conclusions also apply in disjunct or patchy habitats. The influence of errors in positioning is also assessed. The advantages of systematic sampling are reviewed, and more efficient hierarchical approaches are identified. The distinction between biological and statistical significance in all analyses is emphasised.

Animals↗
PubMed1995

Infection risk and potential contamination of urine specimens associated with sample port design of catheter leg bags.

The ease of disinfection of the sample ports of three types of urine drainage leg bags with different sampling port systems was assessed using a bladder bag model. The ports were contaminated with Escherichia coli, 'disinfected' using a standard method, then sampled at time intervals up to one week after contamination. It was discovered that leg bags which employ a needle-based sampling system (the 'sample safe port system') were easier to disinfect than those which did not, and that organisms are retained in large enough numbers to lead to misdiagnosis of a urinary tract infection or to pose a retrograde infection risk.

Catheters, Indwelling↗
PubMed2006

Haplotype reconstruction and estimation of haplotype frequencies from nuclear families with only one parent available.

Recent literature has suggested that haplotype inference through close relatives, especially from nuclear families can be an alternative strategy in determining the linkage phase. In this paper, haplotype reconstruction and estimation of haplotype frequencies via expectation maximization (EM) algorithm including nuclear families with only one parent available is proposed. Parent and his (her) child are treated as parent-child pair with one shared haplotype. This reduces the number of potential haplotype pairs for both parent and child separately, resulting in a higher accuracy of the estimation. In a series of simulations, the comparisons of PHASE, GENEHUNTER, EM-based approach for complete nuclear families and our approach are carried out. In all situations, EM-based approach for trio data is comparable but slightly worse error rate than PHASE, our approach is slightly better and much faster than PHASE for incomplete trios, the performance of GENEHUNTER is very bad in simple nuclear family settings and dramatically decreased with the number of markers being increased. On the other hand, the comparison result of different sampling designs demonstrates that sampling trios is the most efficient design to estimate haplotype frequencies in populations under same genotyping cost.

Child↗
PubMedDate not supplied

Study design and sampling in the Veterans Health Study.

There are numerous choices to be made in the design of studies examining the impact of healthcare on patient-reported outcomes. We describe considerations in the design of the Veterans Health Study (VHS), a large-scale longitudinal observational study of healthcare in the Veterans Health Administration (VA). We also consider sampling issues, and discuss the broader theoretical and practical implications of our choices. The VHS was an observational study with a prospective longitudinal design. Subjects were recruited from a cross-sectional sample of the VA patient population, and identified when they came to ambulatory care clinics for a medical visit. Participating patients were contacted by telephone, and scheduled for an interview conducted at the clinic. Prior to the interview they completed a mailed questionnaire. The clinic interview included brief clinical assessments of selected study medical conditions, a medical history interview, limited health examination, and assessments of health status, health-related quality of life, process-of-care measures related to utilization of services, and other patient characteristics. Patients were empaneled and followed over time. Their health was monitored with brief mailed questionnaires completed at 3-month intervals, and with annual patient reassessments at 12 and 24 months. This design had several strengths. Its comprehensiveness and observational nature allowed for examination of a broad range of outcomes and processes of care as they occur in routine practice in the VA system. Study effects on outcomes should be minimal and the longitudinal design permitted the examination of changes in health status and evaluation of the extent to which changes in patients' illnesses and their treatments were associated with changes in outcomes. Many aspects of this study's design were innovative, reflecting careful consideration of design choices and lessons learned from previous outcomes research studies. Choices made in the design of the VHS can serve as models for future studies of the effects of healthcare on patient-reported outcomes.

Adult↗
PubMed2000

Design and estimation for the National Health Interview Survey, 1995-2004.

OBJECTIVES: This report presents an overview, a detailed description of the sample design features, and estimation structures for the 1995-2004 National Health Interview Survey (NHIS). It is intended to serve the same role for the current (1995-2004) National Health Interview Survey design as the NCHS publication, Series 2, No. 110, Design and Estimation for the National Health Interview Survey, 1985-94, did for the previous design. METHODS: The 1995-2004 NHIS sample design uses cost-effective complex-sampling techniques including stratification, clustering, and differential sampling rates to achieve several objectives. These objectives include improved reliability of racial, ethnic, and geographical domains. This report provides a description of those methods. RESULTS: This report presents the operating characteristics of the 1995-2004 NHIS. The general sampling structure is presented along with a discussion of the weighting and variance estimation techniques. This report is intended for the general users of NHIS data systems. A companion report, Series 2, No. 126, National Health Interview Survey: Research for the 1995-2004 Redesign, provides a finer level of detail on the redesign process.

Data Interpretation, Statistical↗
PubMed2003

Implementation and applications of bootstrap methods for the National Immunization Survey.

In complex probability sample surveys, numerous adjustments are customarily made to the survey weights to reduce potential bias in survey estimates. These adjustments include sampling design (SD) weight adjustments, which account for features of the sampling plan, and non-sampling design (NSD) weight adjustments, which account for non-sampling errors and other effects. Variance estimates prepared from complex survey data customarily account for SD weight adjustments, but rarely account for all NSD weight adjustments. As a result, variance estimates may be biased and standard confidence intervals may not achieve their nominal coverage levels. We describe the implementation of the bootstrap method to account for the SD and NSD weight adjustments for complex survey data. Using data from the National Immunization Survey (NIS), we illustrate the use of the bootstrap (i). for evaluating the use of standard confidence intervals that use Taylor series approximations to variance estimators that do not account for NSD weight adjustments, (ii). for obtaining confidence intervals for ranks estimated from weighted survey data, and (iii). for evaluating the predictive power of logistic regressions using receiver operating characteristic curve analyses that account for the SD and NSD adjustments made to the survey weights.

Analysis of Variance↗
PubMed1995

Design of sampling plans for mycotoxins in foods and feeds.

The control of the occurrence of mycotoxins in foods and feeds requires effective surveillance and quality control procedures which facilitate the identification and control of the mycotoxin problem respectively. Surveillance and quality control procedures involve a sequence of sampling, sample preparation, and analysis steps; and the integrity of the data produced by these procedures will be determined by the effectiveness of these steps. It is imperative that the sampling step is performed as accurately as possible so that the sample collected is representative of the batch of food or feed under investigation. Needless to say, the collection of a biased sample will completely invalidate the resultant analytical data. Most attempts to develop effective sampling protocols have focused upon the aflatoxins, since the majority of current regulations are concerned specifically with this group of mycotoxins. However, the design of effective sampling protocols has been severely hindered by the highly skewed distribution of the aflatoxins in foods and feeds. Studies already performed indicate that representative samples of commodities, composed of large particles (e.g., corn and oilseed kernels) should be 10 kg in weight, at least, and composed of approximately one hundred incremental samples. Similar studies have indicated that samples of oilseed cakes and meal, however, should be composed of fifty incremental samples which afford a composite sample of approximately 5 kg in weight.

Food Analysis↗