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At least 217 records · Page 12Linked to original sources
PubMed2000

Parametric empirical Bayes estimates of disease prevalence using stratified samples from community populations.

Studies of chronic diseases in a community setting often employ stratified sample designs to enable the study to attain multiple research goals at a reasonable cost. One important goal is estimation of disease prevalence in the whole community and in important subgroups. Some adjustment for the sample design is necessary; if the design has many strata with very disparate sampling fractions, simply upweighting observed stratum prevalences may lead to unstable estimators. We propose a parametric empirical Bayes estimator in the spirit of the work of Efron and Morris, and we compare it to the direct upweighted estimator and a regression-smoothed estimator. Simulation studies in realistic settings suggest that the new estimator performs best, giving estimates with low bias and good precision under a variety of models.

Age Factors↗
PubMedDate not supplied

Overview of important design issues for a National Human Exposure Assessment Survey.

Exposure issues have important consequences for regulatory decisions. Reliable answers to exposure questions are critical for site cleanup, model validation, and cumulative risk issues, as well as giving perspective on our risk estimates. This paper discusses some of the important issues in designing the National Human Exposure Assessment Survey (NHEXAS) and, by implication, other exposure-monitoring-based studies as well. Sampling design issues are discussed in terms useful to exposure assessors. These issues include simple random sample designs versus more complex multistage designs, design efficiency, how to determine the sample size for the desired precision of the estimate, and the effects of stratification and oversampling on the needed sample size. This paper also discusses several important nonsampling issues such as population definition, response rates, and several potential sources of error in interpreting the monitoring results.

Data Collection↗
PubMed2001

Design evaluation for a population pharmacokinetic study using clinical trial simulations: a case study.

Clinical trial simulations were conducted to assess power and sample size requirements for a population pharmacokinetic (PK) substudy of a phase III clinical trial. The simulations were based on a population PK model developed from phase I healthy volunteer data. A sparse sampling design was employed taking into account the practical considerations regarding the desire not to keep patients at the study sites for extended periods of time for blood sampling. It was expected that the sparse sampling design would not support fitting the same model developed in healthy volunteers due to the narrow range of sampling times. Therefore, a model with fewer parameters and variance components was fit to simulated data from the proposed design to assess the bias in the estimates of the population mean PK parameters and variance components. Results indicate that the proposed design employing the simple model can provide accurate mean estimates of oral drug clearance (CL) and the apparent steady-state volume of distribution (V(ss)). However, the simulation results also suggest that the size and power of the likelihood ratio test for subpopulation differences in CL are inflated when using the simple model.

Clinical Trials, Phase I as Topic↗
PubMed2001

Effects of age on validity of self-reported height, weight, and body mass index: findings from the Third National Health and Nutrition Examination Survey, 1988-1994.

OBJECTIVE: To compare self-reported to measured heights and weights of adults examined in the Third National Health and Nutrition Examination Survey (NHANES III), and to determine to what extent body mass index (BMI) calculated from self-reported heights and weights affects estimates of overweight prevalence compared with BMI calculated from measured values. DESIGN: A complex sample design was used in NHANES III to obtain a nationally representative sample of the US civilian, noninstitutionalized population. During household interviews, survey respondents were asked their height and weight. Trained health technicians subsequently measured height and weight using standardized procedures and equipment. SUBJECTS: The analytical sample consisted of 7,772 men and 8,801 women 20 years old and older. STATISTICAL ANALYSES PERFORMED: Only persons with measured and self-reported heights and weights were included in the analysis, and statistical sampling weights were applied. t Tests, Pearson product moment correlation coefficients, sensitivity, and specificity analyses were used to determine the validity of self-reported measurements and prevalence estimates of overweight, defined as BMI of 25 or greater. RESULTS: Age is an important factor in classifying weight, height, BMI, and overweight from self-reports. Statistically significant differences were found for the mean error (measured-self-reported values) for height and BMI that were notably larger for older age groups. For example, the mean error for height ranged from 2.92 to 4.50 cm for women and from 3.06 to 4.29 cm for men, 70 years and older. Despite the high correlation between measured and self-reported data, the prevalence of overweight calculated from measured values was higher than that calculated from self-reported values among older adults. When calculated with self-reported height, BMI was one unit lower than when calculated from measured height for persons > or = 70 years. Specificity was high but sensitivity decreased with increasing age cohorts. Regression equations are provided to determine actual height from self-reported values for older adults. CONCLUSION/APPLICATIONS: Self-reported heights and weights can be used with younger adults, but they have limitations for older adults, ages > or = 60 years. In research studies and in clinical settings involving older adults, failure to measure height and weight can result in subsequent misclassification of overweight status. Therefore, registered dietitians are encouraged to obtained a measured weight and height using a calibrated scale and stadiometer.

Adult↗
PubMed1997

ICS-II USA research design and methodology.

The purpose of the WHO-sponsored International Collaborative Study of Oral Health Outcomes (ICS-II) was to provide policy-markers and researchers with detailed, reliable, and valid data on the oral health situation in their countries or regions, together with comparative data from other dental care delivery systems. ICS-II used a cross-sectional design with no explicit control groups or experimental interventions. A standardized methodology was developed and tested for collecting and analyzing epidemiological, sociocultural, economic, and delivery system data. Respondent information was obtained by household interviews, and clinical examinations were conducted by calibrated oral epidemiologists. Discussed are the sampling design characteristics for the USA research locations, response rates, samples size for interview and oral examination data, weighting procedures, and statistical methods. SUDAAN was used to adjust variance calculations, since complex sampling designs were used.

Adult↗
PubMed2006

Adaptive web sampling.

A flexible class of adaptive sampling designs is introduced for sampling in network and spatial settings. In the designs, selections are made sequentially with a mixture distribution based on an active set that changes as the sampling progresses, using network or spatial relationships as well as sample values. The new designs have certain advantages compared with previously existing adaptive and link-tracing designs, including control over sample sizes and of the proportion of effort allocated to adaptive selections. Efficient inference involves averaging over sample paths consistent with the minimal sufficient statistic. A Markov chain resampling method makes the inference computationally feasible. The designs are evaluated in network and spatial settings using two empirical populations: a hidden human population at high risk for HIV/AIDS and an unevenly distributed bird population.

Animals↗
PubMed1991

Assessment of long-term exposures to toxic substances in air.

Because airborne exposure varies greatly over time and between individual workers, occupational hygienists should adopt sampling strategies which recognize the inherent statistical nature of assessing exposure. This analysis indicates that the traditional practice of testing 'compliance' with occupational exposure limits (OELs) should be discarded. Rather, it is argued that acceptable exposure should be defined with reference to the exposure distribution. Regarding the many statistical issues which come into play, it is concluded that hygienists should continue to apply the log-normal model for summarizing and testing data. However, sampling designs should move away from methods which are biased (e.g. sampling only the worst case) and which rely upon job title and observation as the primary means of assigning workers into groups. Since exposure data often lack independence (e.g. owing to the autocorrelation of serial measurements) and there exist large differences in exposure between workers in the same job group, random sampling designs should be adopted. It is also shown that the relationship between the mean of a log-normal distribution and exposures in the right tail allows one to evaluate simultaneously the mean exposure and the maximum frequency with which exposures exceed the OEL. Investigation of the biological concepts relies heavily upon a conceptual model which depicts the exposure-response continuum as a sequence of time series related to exposure, burden, damage and risk. Analysis of the linkages between these processes identifies two kinetic conditions which are necessary if variability of exposure is to affect appreciably the individual's risk of chronic disease. First, the variation of exposure from interval to interval must be efficiently translated into burden and damage (no damping), and second, during periods of intense exposure the relationship between burden and damage must be non-linear (curving upwards). On the basis of current knowledge it appears that relatively few chronic toxicants satisfy both these conditions. Even for those substances which cause damage only when a threshold is exceeded, a statistical argument suggests that the maximum risk can still be related to the mean exposure received over time. It is concluded that the risk of chronic disease generally depends upon the mean exposure received by the individual worker over time. Thus, the sampling strategy must allow the distribution of individual mean exposures to be characterized across the population at risk. It follows from this paradigm for assessing exposures that relatively little effort should be devoted to the evaluation of short-term 'peak' exposures since such transients are unlikely to exert undue influence on long-term effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Air Pollutants, Occupational↗
PubMed1997

Design for sample size re-estimation with interim data for double-blind clinical trials with binary outcomes.

Estimation of sample size in clinical trials requires knowledge of parameters that involve the treatment effect and variability, which are usually uncertain to medical researchers. The recent release within the European Union of a Note for Guidance from the Commission for Proprietary Medical Products (CPMP) highlights the importance of this issue. Most previous papers considered the case of continuous response variables that assume a normal distribution; some regarded the portion up to the interim stage as an 'internal pilot study' and required unblinding. In this paper, our concern is with the case of binary response variables, which is more difficult than the normal case since the mean and variance are not distinct parameters. We offer a design with a simple stratification strategy that enables us to verify and update the assumption of the response rates given initially in the protocol. The design provides a method to re-estimate the sample size based on interim data while preserving the trial's blinding. An illustrative numerical example and simulation results show slight effect on the type I error rate and the decision making characteristics on sample size adjustment.

Clinical Trials as Topic↗
PubMed2000

Integrated sample collection and handling for drug discovery bioanalysis.

An integrated sample handling process for drug discovery bioanalysis is described. The streamlining of study design, sample collection and automatic bioanalytical sample processing is demonstrated. Specific details for the entire procedure regarding the time saved, ease of automation and integration are defined. Details of sample handling involved a sample collection map, sample collection formatting and volume, dilution schemes for high concentration samples, choice of biological fluid and evaluating the capabilities of two liquid-handling workstations. Numerous comparisons were conducted between the new approaches and the conventional sample handling approaches. The precision and accuracy obtained from the new integrated sample handling process were comparable to those obtained from a conventional approach, as were pharmacokinetic profiles and parameters. This new sampling process greatly improved the efficiency of drug discovery bioanalysis. The integration of pre-clinical protocol design, sample collection and bioanalysis processes was also achieved.

Animals↗
PubMed2003

Spatial indoor radon distribution in Mexico City.

We present a spatial analysis of residential radon concentrations in the Mexico City Metropolitan Area, which we intend to use to assign radon exposure in an ongoing case-control study. As part of a probabilistic household survey, carried out between May and June 1999, 501 dwellings were selected for indoor placement of solid state nuclear track detectors (LR 115) in a cup array over a period of approximately 90 days. As part of the sampling design, the city was grid partitioned into nine zones and a sample of dwellings was selected in each zone. All zones were simultaneously surveyed. The stratified sampling design allowed us to obtain radon geometric means, adjusted for household characteristics, week of detector placement and number of days of measurement for these zones. Additionally, adjusted geometric means were estimated for the 100 census tracts surveyed and this information was used to obtain a more detailed spatial distribution of residential radon levels through kriging interpolation and surface contouring. Radon levels depended on the room of placement, the floor level and the ventilation habits but not on building materials. Regarding the city zone, the highest adjusted geometric mean was found in the southwest (136 Bqm(-3)), where 46% of the households had an estimated radon level in excess of 200 Bqm(-3). In the rest of the city, the geometric mean concentration ranged between 41 and 98 Bqm(-3). A more detailed spatial distribution showed that, in general, most of the eastern and middle zones of the city had estimated radon geometric means below 74 Bqm(-3), while the western ones had geometric means above this concentration. Very high geometric means, exceeding 111 Bqm(-3) and even reaching 288 Bqm(-3), are estimated for some areas located in the southern and western zones of Mexico City. The obtained spatial distribution shows that the areas with very high estimated residential radon concentrations are close to inactive volcanic mountains. We believe that the geo-statistical techniques, we have used, offer reasonably good estimates of the average spatial residential radon distribution in Mexico City under average ventilation in homes. The use of this indirect approach for radon exposure measurement in epidemiological studies is an inexpensive alternative to direct radon exposure measurement but may be subject to non-differential misclassification error. The effect of such error on the detection of a real increase in lung cancer risk from indoor radon remains to be determined.

Air Pollution, Indoor↗
PubMed2005

An evaluation of an optimal sampling strategy for meropenem in febrile neutropenics.

Optimal sampling design with nonparametric population modeling offers the opportunity to determine pharmacokinetic parameters for patients in whom blood sampling is restricted. This approach was compared to a standard individualized modeling method for meropenem pharmacokinetics in febrile neutropenic patients. The population modeling program, nonparametric approach of expectation maximization (NPEM), with a full data set was compared to a sparse data set selected by D-optimal sampling design. The authors demonstrated that the D-optimal sampling strategy, when applied to this clinical population, provided good pharmacokinetic parameter estimates along with their variability. Four individualized and optimally selected sampling time points provided the same parameter estimates as more intensive sampling regimens using traditional and population modeling techniques. The different modeling methods were considerably consistent, except for the estimation of CL(d) with sparse sampling. The findings suggest that D-optimal sparse sampling is a reasonable approach to population pharmacokinetic/pharmacodynamic studies during drug development when limited sampling is necessary.

Anti-Bacterial Agents↗
PubMed2003

[Estimates from a complex survey].

OBJECTIVE: To evaluate the impact of sampling design and the effect of weighting on data from the 1996 Brazilian National Survey on Demography and Health. METHODS: Secondary data analysis was performed using a sample of 1,355 interviewed women of the state of São Paulo. The sampling design of the National Survey of Household Sampling (PNAD) was used as a reference, and the municipality as primary sampling unit. The ratio estimator and Taylor's approximation for variance were calculated using the primary sampling units and several modalities of weighting. The indicators used to evaluate precision and validity were confidence intervals, design effects (Deff) and biases. RESULTS: For the four procedures, the differences between upper and lower point estimates for prevalence were not greater than 10%. The differences on ranges of confidence intervals were less than 20%. Use of condom and hormone injection were the variables that showed design effects greater than 1.5 and biases greater than 0.20. CONCLUSIONS: According to the results, it could be said that the cluster sampling had an impact on the precision of the estimates for two out of six variables. The impact of weighting was not significant.

Bias↗
PubMed2001

The relationship between pharmacists' tenure in community setting and moral reasoning.

OBJECTIVE: To explore the relationship between pharmacists' tenure in the community setting and their moral reasoning abilities. DESIGN: Systematic random sample design. SETTING: A large southeastern city in the United States. PARTICIPANTS: 450 independent and chain community pharmacists identified from the state board of pharmacy list of licensed community pharmacists. INTERVENTIONS: A mailed questionnaire that included a well-known moral reasoning instrument and collected demographic information. MAIN OUTCOME MEASURES: Moral Reasoning abilities and tenure of community pharmacists. RESULTS: As a group, community pharmacists with greater years of tenure in community practice scored significantly lower on moral reasoning than those pharmacists with fewer years of tenure (p=0.016). CONCLUSION: Four plausible explanations for the results are given including: a) a selection of lower ethical reasoners and/or an exodus of higher ethical reasoners from the community setting; b) a retrogression in the moral reasoning skills as community pharmacists obtain tenure in this setting; c) differences between the low and high moral reasoning groups may be due to a cohort effect; and d) the obtained practitioner sample may not have been representative of the population of community pharmacists.

Community Pharmacy Services↗
PubMed1991

Sequential or fixed sample trial design? A case study by stochastic simulation.

The properties of Wilcoxon's rank sum test for fixed sample size and a Wilcoxon-type two-sample sequential test have been illustrated and compared by means of stochastic simulation. Data from a real fixed sample trial have been used, both for resampling from the original data, and for construction of an idealized theoretical distribution. The sequential and the fixed sample test obtain equal power, but the sequential test mostly includes considerably fewer patients to reach a conclusion, i.e. the mean and median number of patients included are both much lower than the fixed sample size. Under the hypotheses only a small fraction of the simulation runs exceed the fixed sample size. These findings exemplify results obtained in theoretical analyses and simulation studies covering a wide range of distributions. In our opinion sequential tests have obvious advantages and are in many cases better alternatives than fixed sample tests in clinical trials.

Clinical Trials as Topic↗
PubMed1994

Evaluating peer reviews. Pilot testing of a grading instrument.

OBJECTIVE: To measure the reliability and preliminary validity of a grading instrument for editors to evaluate the quality of peer reviews. DESIGN: The consecutive sample design included 53 reviews of 23 manuscripts. Reviews were systematically assigned to interrater reliability (n = 41; power greater than 0.90 to detect a difference of greater than one point) and preliminary criterion-related validity (n = 12) subsamples. Content validity was closely examined. SETTING: Nonclinical. PARTICIPANTS: Three graders evaluated reliability. One individual examined content validity and two editors tested preliminary criterion-related validity. INTERVENTION (INSTRUMENT)--Attributes reflecting two basic dimensions, review content and format, were identified and scored (values are possible points/percent contribution): timeliness, 3/21%; grade sheet, 1/7%; etiquette, 1/7%; sectional narratives, 3/21%; citations, 2/14%; narrative summary, 2/14%; and insights, 2/14%. A scoring guide was provided. MAIN OUTCOME MEASURES: Statistical analyses used to test the interrater reliability of the total score included the intraclass correlation coefficient and analysis of variance with the expectation to uphold the null hypothesis. Kendall's coefficient of concordance was used to test preliminary criterion-related validity. RESULTS: The intraclass correlation coefficient was .84 (P < .001) and a lack of difference between mean scores was demonstrated by analysis of variance (P = .46). Content validity was confirmed and preliminary criterion-related validity was indicated (Kendall's coefficient of concordance = .94, P = .038). CONCLUSIONS: The instrument is reliable. Content validation has been completed, and further criterion-related validation is warranted.

Evaluation Studies as Topic↗
PubMed2006

Co-occurrence of mental and physical illness in US Latinos.

BACKGROUND: This study describes the prevalence of comorbid physical and mental health problems in a national sample of US Latinos. We examined the co-occurrence of anxiety and depression with prevalent physical chronic illnesses in a representative sample of Latinos with national origins from Mexico, Cuba, Puerto Rico, and other Latin American countries. METHOD: We used data on 2,554 Latinos (75.5% response rate) ages 18 years and older from the National Latino and Asian American Study (NLAAS). The NLAAS was based on a stratified area probability sample design, and the sample came from the 50 states and Washington, DC. Survey questionnaires were delivered both in person and over the telephone in English and Spanish. Psychiatric disorders were assessed using the World Mental Health Survey Initiative version of the World Health Organization Composite International Diagnostic Interview (WMH-CIDI). Physical chronic illness was assessed by self-reported history. RESULTS: Puerto Ricans had the highest prevalence of meeting criteria for any comorbid psychiatric disorder (more than one disorder). Puerto Ricans had the highest prevalence (22%) of subject-reported asthma history, while Cubans had the highest prevalence (33%) of cardiovascular disease. After accounting for age, sex, household income, number of years in the US, immigrant status, and anxiety or depression, anxiety was associated with diabetes and cardiovascular disease, in the entire sample. Depression and co-occurring anxiety and depression were positively associated with having a history of asthma but not with other physical diseases, in the entire sample. Interestingly, Puerto Ricans with a depressive disorder had a lower odds of having a history of cardiovascular disease than Puerto Ricans without a depressive disorder. The relationship between chronic physical and mental illness was not confounded by immigration status or number of years in the US. DISCUSSION: Despite previous findings that link acculturation with both chronic physical and mental illness, this study does not find that number of years in the US nor nativity explain the prevalence of psychiatric-medical comorbidities. This study demonstrates the importance of considering psychiatric and medical comorbidity among specific ethnic groups, as different patterns emerge than when using aggregate ethnic measures. Research is needed on both the pathways and the mechanisms of comorbidity for the specific Latino groups.

Adult↗
PubMed1988

D-optimal design applied to binding saturation curves of an enkephalin analog in rat brain.

The D-optimal design, a minimal sample design that minimizes the volume of the joint confidence region for the parameters, was used to evaluate binding parameters in a saturation curve with a view to reducing the number of experimental points without loosing accuracy in binding parameter estimates. Binding saturation experiments were performed in rat brain crude membrane preparations with the opioid mu-selective ligand [3H]-[D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO), using a sequential procedure. The first experiment consisted of a wide-range saturation curve, which confirmed that [3H]-DAGO binds only one class of specific sites and non-specific sites, and gave information on the experimental range and a first estimate of binding affinity (Ka), capacity (Bmax) and non-specific constant (k). On this basis the D-optimal design was computed and sequential experiments were performed each covering a wide-range traditional saturation curve, the D-optimal design and a splitting of the D-optimal design with the addition of 2 points (+/- 15% of the central point). No appreciable differences were obtained with these designs in parameter estimates and their accuracy. Thus sequential experiments based on D-optimal design seem a valid method for accurate determination of binding parameters, using far fewer points with no loss in parameter estimation accuracy.

Animals↗
PubMed2006

Macroprolactin, big-prolactin and potential effects on the misdiagnosis of hyperprolactinemia using the Beckman Coulter Access Prolactin assay.

OBJECTIVE: To examine whether use of the Beckman Coulter Access Prolactin (PRL) assay, which has low reactivity with macro-PRL, obviates the need for screening hyperprolactinemic samples. DESIGN AND METHODS: Samples from 1020 hyperprolactinemic individuals and 401 healthy volunteers were treated with polyethylene glycol (PEG). Macro-PRL was assessed from (1) percent PRL recovery, using cut-off values derived by gel filtration chromatography (GFC) and (2) significant (p<0.05) normalisation of PRL following PEG. RESULTS: PRL recovery was similar in volunteer and hyperprolactinemic samples (mean+/-SD 101+/-13% and 101+/-19%, respectively). In hyperprolactinemic samples, macro-PRL was identified from PRL recovery in 9.7%, although levels were moderate to high in only 3.9%. The total PRL normalised following PEG in 7.4%. Correlations of PRL recovery with the proportions of macro-, big- and monomeric PRL following GFC (n=30 samples, range of PRL and macro-PRL levels) were -0.89, -0.20 and 0.92, respectively. The big-PRL content was 0-28%. Regression analysis suggested that PEG precipitated both macro-PRL and big-PRL. CONCLUSIONS: Using the Access assay, macro-PRL can cause apparent hyperprolactinemia and big-PRL may cause misclassification of individuals. Screening using PEG is applicable to assays with low macro-PRL reactivity provided specific reference values are derived.

Chromatography, Gel↗