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PubMed1991

Selecting an efficient design for assessing exposure-disease relationships in an assembled cohort.

We develop approximate methods to compare the efficiencies and to compute the power of alternative potential designs for sampling from a cohort before beginning to collect exposure data. Our methods require only that the cohort be assembled, meaning that the numbers of individuals Nkj at risk at pairs of event times tk and tj greater than or equal to tk are available. To compute Nkj, one needs to know the entry, follow-up, censoring, and event history, but not the exposure, for each individual. Our methods apply to any "unbiased control sampling design," in which cases are compared to a random sample of noncases at risk at the time of an event. We apply our methods to approximate the efficiencies of the nested case-control design, the case-cohort design, and an augmented case-cohort design, compared to the full cohort design, in an assembled cohort of 17,633 members of an insurance cooperative who were followed for mortality from prostatic cancer. The assumptions underlying the approximation are that exposure is unrelated both to the hazard of an event and to the hazard for censoring. The approximations performed well in simulations when both assumptions held and when the exposure was moderately related to censoring.

Biometry↗
PubMed2003

Factors influencing the design of bioaccumulation factor and biota-sediment accumulation factor field studies.

A series of modeling simulations was performed to evaluate the underlying factors and principles that drive the uncertainty in measured bioaccumulation factors (BAFs) and biota-sediment accumulation factors (BSAFs) for fish, and to determine which sampling designs minimize those uncertainties. Temporal variability of chemical concentrations in the water column, and the metabolism rate and n-octanol-water partition coefficient (Kow) for the chemical were determined to be the dominant factors that influenced the field-sampling design. Spatial variability of the chemical concentrations, food web structure, and the sediment-water column concentration quotient had a lesser importance upon the overall design. The simulations also demonstrated that collection of composite water samples in comparison to grab water samples resulted in reductions in the uncertainties associated with measured BAFs. Some illustrative sampling design structures for BAF and BSAF measurements based upon the temporal variability of chemical concentrations in the water column and the metabolism rate and Kow for the chemical were developed. These illustrative designs provided a sense of how sampling design structures, that is, the number and spacing over time of sampling events, might be influenced by differences in temporal variabilities, metabolism rates, and Kow. Although the importance of spatial variability was discounted as a major factor in the design process, sampling of water and sediment across the immediate home range of the sampled organism is required for successful measurements because poor spatial coordination of organism, water, or sediment samples will yield BAFs and BSAFs with large and unknown biases.

Animals↗
PubMed2002

Use of out-of-hospital variables to predict severity of injury in pediatric patients involved in motor vehicle crashes.

STUDY OBJECTIVE: We sought to create a clinical decision rule, on the basis of variables available to out-of-hospital personnel, that could be used to accurately predict severe injury in pediatric patients involved in motor vehicle crashes as occupants. METHODS: We analyzed the National Automotive Sampling System database, a national probability sample, using pediatric patients up to 15 years old (occupants only) involved in motor vehicle crashes from January 1993 to December 1999. The National Automotive Sampling System database includes patients from regions throughout the country, weighted to represent a nationwide sample. Twelve out-of-hospital variables were used in classification and regression tree analysis to create a decision rule separating children with severe injuries (Injury Severity Score [ISS] > or =16) from those with minor injuries (ISS < 16). Misclassification costs and complexity parameters were selected to yield a decision tree with appropriate sensitivity and specificity for the identification of severely injured patients, while also being simple and practical for out-of-hospital use. Probability weights were used throughout the analysis to account for the sampling design and sampling weights. RESULTS: Using a sample size of 8,392 children, we constructed a decision rule using 3 out-of-hospital variables (Glasgow Coma Scale score, passenger space intrusion > or =6 in [> or =15 cm], and restraint use) to predict those patients with an ISS of 16 or more. Internal cross-validation was used to determine the sensitivity and specificity, yielding values of 92% and 73%, respectively, for the prediction of patients with an ISS of 16 or more. CONCLUSION: Out-of-hospital variables available to field personnel could be used to effectively triage pediatric motor vehicle crash patients using the decision rule developed here. Prospective trials would be needed to test this decision rule in actual use.

Accidents, Traffic↗
PubMed2002

Ethnic differences in predictors of initiation and persistence of adolescent cigarette smoking in the National Longitudinal Survey of Youth.

AIMS: To identify and compare predictors of adolescent smoking initiation and persistence among African American, Hispanic and White adolescents in a longitudinal national sample. DESIGN: The sample includes 1537 mother-child dyads from the National Longitudinal Survey of Youth (NLSY). Family, youth, peer and sociodemographic risk and protective factors were analyzed. FINDINGS: White adolescents reported the highest rates of smoking initiation and persistence; African Americans and Hispanics the lowest. Multivariate analyses revealed mostly common and few ethnic-specific predictors of smoking initiation and persistence. For initiation, maternal current smoking, child age, child problem behavior, and perceived peer pressure to smoke were predictive across ethnic groups; female gender and ineffective parenting were predictive among Whites only. For persistence, child age, child problem behavior and perceived scholastic competence were predictive across ethnic groups; negative mood was predictive among Whites only. CONCLUSIONS: More common than unique factors predict smoking initiation and persistence among adolescents of different ethnicity. However, the power to detect ethnicity-by-predictor interactions with respect to persistence was low. Social factors are more important for smoking initiation, whereas individual factors are more important for persistence, although child problem behaviors are common determinants both of initiation and persistence. With few exceptions, universal anti-smoking interventions should be targeted to youths of different ethnicity.

Adolescent↗
PubMed2000

The effects of hydrocolloid dressing and gentian violet on radiation-induced moist desquamation wound healing.

The aim of the study was to compare the effect of a gentian violet topical application with that of a moist dressing (hydrocolloid) on the rate and efficacy of radiotherapy-induced moist desquamation wound healing and the patients' satisfaction level with each method. This prospective randomized clinical trial used a stratified sampling design. A sample of 39 patients with 60 wounds had their wounds assessed on alternate days in terms of several wound-healing parameters including wound size, wound pain, incidence of infection, and time required for healing. Patient satisfaction with each treatment was evaluated at the completion of the study. Gentian violet significantly decreased wound size and reduced wound pain. However, this treatment received significantly lower ratings for dressing comfort and dressing aesthetic acceptance. Nevertheless, the time required for healing was not statistically different in the two groups. These findings suggest that the lower score of dressing satisfaction level in the gentian violet group may result from the skin discoloration and drying effects of the treatment, which renders patients unable to move or stretch their skin. Although the aim is to have complete wound healing, this may not be realistic for many lesions such as radiotherapy-induced moist desquamation wounds. The best evidence on which to make decisions about individual care can now be based on patients' own perception of quality.

Analysis of Variance↗
PubMed1998

Muscularity and fatness of infants and young children born small- or large-for-gestational-age.

OBJECTIVE: There is growing interest in the extent to which body composition, both short- and long-term, differs in infants and children born at the extremes of birth weight. This is because a growing number of studies have linked low birth weight and fetal growth restriction to the chronic diseases in adulthood that often are obesity-related, and there is also evidence to suggest that heavy infants may be at increased risk for obesity in later life, again with the attendant obesity-related chronic diseases. Our objective was to compare anthropometric indices of body composition of infants and young children born small-for-gestational-age (SGA, <10th percentile) or large-for-gestational age (LGA, >/=90th percentile) with those of normal birth weight status (appropriate-for-gestational-age, AGA) in a US sample. DESIGN: National sample of US-born non-Hispanic white, non-Hispanic black, and Mexican-American infants and young children, 2 to 47 months of age, examined in the third National Health and Nutrition Examination Survey (NHANES III, 1988-1994), for whom birth certificates were obtained. The primary outcomes were normalized anthropometric indices (z scores or standard deviation units [SDU]) of nutritional status and body composition (mid-upper arm circumference, triceps and subscapular skinfolds, mid-upper arm muscle and mid-upper arm fat areas (UFA), and the arm fat index). The outcomes thus were scaled to permit comparison across chronologic ages. RESULTS: The prevalence of SGA was 8.6%, appropriate-for-gestational-age 80.9%, and LGA 10.5%. From ages 2 to 47 months, for infants and young children born SGA, there was a persistent overall deficit in muscularity (mid-upper arm circumference and mid-upper arm muscle area) of approximately -0.50 SDU, but less of a deficit in fatness, particularly at the youngest ages. For infants and young children born LGA, there was a surfeit in muscularity of approximately 0.45 SDU, with less of a surfeit in fatness, particularly at the youngest ages. Across all ages, the mean UFA showed a statistically significant deficit for SGA children (-0.27 +/- 0.10 SDU) and surfeit for LGA children (0.24 +/- 0.08 SDU). At individual ages for UFA and at individual and all ages combined for skinfold thicknesses, there were no significant differences in level of subcutaneous fatness in the three birth-weight-for-gestational-age groups. There was a tendency in the first year for the arm fat index (% arm fat) to be significantly higher for SGA infants, but the effect did not persist after the first year. CONCLUSION: SGA infants remain smaller and LGA infants larger in size through early childhood, but the discrepancies in weight are primarily attributable to differences in lean body mass (muscularity). Fatness is less affected. Thus, based on the fatness indicators used, at any given weight for infants and children 2 to 47 months of age, percent body fat appears to be relatively higher for children who were SGA at birth and lower in those who were LGA at birth. These differences in body composition for SGA infants support the evidence documenting a link between disturbances in intrauterine growth and chronic disease associated with subsequent adiposity in adulthood.

Adipose Tissue↗
PubMed1999

[Process and results of field work concerning the Federal Health Survey].

"I + G Gesundheitsforschung" was responsible for data acquisition and data processing of the survey, which included the production of the survey documents and the conceptualization and production of the operation manual. Furthermore, the institute's task was sample design and sample drawing. This article reports on the continuity and the results of the field work and includes experiences and realizations resulting from the field work. These might be helpful for similar surveys in the future and contribute to the improvement of the procedures.

Adolescent↗
PubMed2003

Illicit drug use among Canadian University undergraduates.

The purpose of this study was to examine rates and patterns of illicit drug use among Canadian university undergraduates, to compare these rates with those for non-university samples, and to describe drug-use trends among university undergaduates in the province of Ontario between 1988 and 1998. A national mail survey was carried out based on stratified 2-stage sample design. The sample comprised 7,800 Canadian undergraduates from 16 universities (52% of eligible respondents). Approximately 47.5% reported use of illicit drug durning their life, 29.6% in the previous 12 months, and 18.7% since the beginning of the academic year. Cannabis was by far the most widley used drug (47.0%, 28.7%, and18.2%, respectively). Many of the gender and regional associations were similar to those found in general-populations surveys. Comparisons to non-university peers did not indicate elevated rates among university students. Among Ontario university undergraduates the use of cannabis, hallucinogens, methamphetamines, crack, and herion remained stable between 1988 and 1998. The use of cocaine declined from 4.8% to 1.7%. Rates of illicit drug use were not appreciably higher than those among their non-university peers. Other public-health issues, such as heavy drinking and poor mental health, override those related to illicit drug use.

Adult↗
PubMed2003

Estimation of population pharmacokinetic parameters in the presence of non-compliance.

In population pharmacokinetic (PK) studies, patients' drug plasma profiles are routinely analyzed assuming that all patients took their drug at the times and in the amounts specified. However, patient non-compliance with the prescribed drug regimen is a leading source of failure to drug therapy. It has been reported that over 30% of patients routinely skip doses regardless of their disease, prognosis, or symptoms. This brings into question the assumption regarding full compliance for population PK analyses. This paper describes the estimation of population PK parameters in the presence and absence of non-compliance while either assuming full compliance or estimating compliance using a hierarchical Bayesian approach. Assessment of compliance for a given dose was limited to one of three possibilities: no dose was taken at the prescribed time, the prescribed dose was taken at the prescribed time, or twice the prescribed dose was taken at the prescribed time. Simulated data sets based on a one-compartment pharmacokinetic model with first order elimination were analyzed using WinBUGS (Bayesian inference Using Gibbs Sampling) software. An initial feasibility simulation experiment, using a simple, but informative PK sampling design with bolus input of drug, was performed. A second simulation study was then carried out using a more realistic sampling design and first-order input of drug. The simulated sampling design included observations after known doses as well as after uncertain doses. Results from the feasibility study revealed that when compliance was estimated instead of being assumed to be 100%, the relative prediction error for clearance (CL) decreased from 0.25 to 0.10 for 60% compliance and from 0.6 to 0.2 for 35% compliance. Estimates of the interoccasion variability of clearance were improved by compliance estimation but still had substantial positive bias. Estimated of interindividual variability were relatively insensitive to compliance estimation. Estimates for volume of distribution (V) and its associated variances were not affected by incorporation of compliance estimates, perhaps due to the specific sampling design that was used. The design was relatively uninformative regarding V. In the more realistic study, estimates for CL, V and the difference between the absorption rate constant and the elimination rate constant (KA-K) were improved by the incorporation of compliance estimation. The median relative errors were reduced from 0.51 to -0.01 for CL, from 0.49 to 0.04 for V, and from 0.49 to -0.02 for Ka-K. The bias in interoccasion variances for V and CL appeared to be reduced by compliance estimation while estimates of interindividual variability were not affected in a systematic fashion. The bias in the residual error variance was decreased from a relative error of about 2 to close to 0. The use of hierarchical Bayesian modeling with the incorporation of compliance estimation decreased the bias in the typical value parameter but the effects on variance parameters were less consistent. The encouraging results of these simulation experiments will hopefully stimulate further evaluation of this methodology for the estimation of population pharmacokinetic parameters in the presence of potential patient noncompliance.

Bayes Theorem↗
PubMed1996

Study design and sample size considerations for half-life studies.

Most studies on the half-lives of environmental contaminants have been based on small sample sizes and a limited number of repeated measurements. In this paper, we address issues of study design and sample size for half-life studies. Useful guidelines are provided for choosing the number of repeats and the optimal time interval between repeats for estimating an individual's half-life with a given level of precision, while minimizing the cost of the study. In addition, sample size and power considerations for studies comparing two population half-lives are investigated. An example is presented using data from a study on polychlorinated biphenyls and breast cancer.

Breast Neoplasms↗
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↗
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↗