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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↗

PubMed2006

Studies on the nutritional status of children aged 0-5 years in a drought-affected desert area of western Rajasthan, India.

OBJECTIVE: The present study was undertaken to assess the impact of drought on the nutritional status of pre-school children aged 0-5 years from a rural population in a desert area facing drought conditions very frequently. DESIGN: The sampling design for assessment was the three-stage sampling technique. SETTING: The study was carried out in 24 villages belonging to six tehsils (sub-units of district) of Jodhpur District, a drought-affected desert district of western Rajasthan, during a drought in 2003. SUBJECTS: A total of 914 children were examined at household level, with nutritional status assessed by anthropometry, dietary intake and clinical signs of nutritional deficiency. RESULTS: The results revealed growth retardation. Stunting (malnutrition of long duration) was observed in 53% of children and underweight in 60%. Wasting, an indicator of short-duration malnutrition, was present in 28% of children. The extent of malnutrition was significantly higher in girls than boys (P<0.05). Vitamin A and B complex deficiencies were found in 0.7 and 3.0% of children, respectively. Prevalence of marasmus (protein-energy malnutrition, PEM) was 1.7% (2.3% in boys and 1.1% in girls). Overall deficits in mean energy and protein intakes were very high (76 and 54%, respectively). Comparison of the present drought results with earlier studies in desert normal and desert drought conditions showed higher prevalence of PEM and higher dietary energy and protein deficiencies. CONCLUSIONS: The prevalence of wasting was high, greater than the cut-off point of 15% stated by the World Health Organization to indicate that the severity of malnutrition is critical. PEM, vitamin A and B complex deficiencies and anaemia, along with dietary deficits of energy and protein, were observed to be higher than in non-desert areas. This may be due to the harsh environmental conditions in desert areas where drought occurs quite frequently and adversely affects the economy, largely by eroding the coping capacity and economic potential of the people as a result of heavy livestock losses and reduced harvests, leading to increased poverty and poor food intake of the inhabitants. Due to inadequate consumption of daily food the children were suffering from wasting and PEM. Efforts should be made to incorporate measures, such as ensuring the supply of adequate energy and protein to all age groups and especially pre-school children, into ongoing nutrition programmes in order to improve the food security of local inhabitants in this area.

Avitaminosis↗

PubMed2006

Rapid determination of protein solubility and stability conditions for NMR studies using incomplete factorial design.

Sample preparation constitutes a crucial and limiting step in structural studies of proteins by NMR. The determination of the solubility and stability (SAS) conditions of biomolecules at millimolar concentrations stays today empirical and hence time- and material-consuming. Only few studies have been recently done in this field and they have highlighted the interest of using crystallogenesis tools to optimise sample conditions. In this study, we have adapted a method based on incomplete factorial design and making use of crystallisation plates to quantify the influence of physico-chemical parameters such as buffer pH and salts on protein SAS. A description of the experimental set up and an evaluation of the method are given by case studies on two functional domains from the bacterial regulatory protein LicT as well as two other proteins. Using this method, we could rapidly determine optimised conditions for extracting soluble proteins from bacterial cells and for preparing purified protein samples sufficiently concentrated and stable for NMR characterisation. The drastic reduction in the time and number of experiments required for searching protein SAS conditions makes this method particularly well-adapted for a systematic investigation on a large range of physico-chemical parameters.

Bacterial Proteins↗

PubMed2004

Demographic and socioeconomic factors associated with disparity in wheelchair customizability among people with traumatic spinal cord injury.

OBJECTIVES: To determine if a standard of care for wheelchair provision exists within the participating centers and if there is disparity in wheelchair customizability among the study sample. DESIGN: Convenience sample survey. SETTING: Thirteen Model Spinal Cord Injury Systems that provide comprehensive rehabilitation for people with traumatic spinal cord injury (SCI) and that are part of the national database funded through the US Department of Education. PARTICIPANTS: A total of 412 people with SCI who use wheelchairs over 40 hours a week. INTERVENTION: Survey information was obtained from subjects via telephone and in-person interviews and from the national database. Collected information included age, race, education, level of injury, and wheelchair funding source. MAIN OUTCOME MEASURES: Number and type (manual or power) of wheelchairs. Wheelchair customizability as defined by design features (eg, adjustable axle position, programmable controls). RESULTS: Ninety-seven percent of manual wheelchair users and 54% of power wheelchair users had customizable wheelchairs. No power wheelchair user received a wheelchair without programmable controls. Minorities with low socioeconomic backgrounds (low income, Medicaid/Medicare recipients, less educated) were more likely to have standard manual and standard programmable power wheelchairs. Older subjects were also more likely to have standard programmable power wheelchairs. CONCLUSIONS: The standard of care for manual wheelchair users with SCI is a lightweight and customizable wheelchair. The standard of care for power wheelchairs users has programmable controls. Unfortunately, socioeconomically disadvantaged people were less likely to receive customizable wheelchairs.

Activities of Daily Living↗

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↗

PubMed2007

Evaluation of the feasibility of international growth standards for school-aged children and adolescents.

The development of an international growth standard for the screening, surveillance, and monitoring of school-aged children and adolescents has been motivated by 2 contemporaneous events, the global surge in childhood obesity and the release of a new international growth standard for infants and preschool children by the WHO. If a prescriptive approach analogous to that taken by WHO for younger children is to be adopted for school-aged children and adolescents, several issues need to be addressed regarding the universality of growth potential across populations and the definition of optimal growth in children and adolescents. A working group of experts in growth and development and representatives from international organizations concluded that subpopulations exhibit similar patterns of growth when exposed to similar external conditioners of growth. However, based on available data, we cannot rule out that observed differences in linear growth across ethnic groups reflect true differences in genetic potential rather than environmental influences. Therefore, the sampling frame for the development of an international growth standard for children and adolescents must include multiethnic sampling strategies designed to capture the variation in human growth patterns. A single international growth standard for school-aged children and adolescents could be developed with careful consideration of the population and individual selection criteria, study design, sample size, measurements, and statistical modeling of primary growth and secondary ancillary data. The working group agreed that existing growth references for school-aged children and adolescents have shortcomings, particularly for assessing obesity, and that appropriate growth standards for these age groups should be developed for clinical and public health applications.

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↗

PubMedDate not supplied

[Profile of sexual behavior in 12- to 19-year-old Mexican adolescents. Results of ENSA 2000].

OBJECTIVE: To describe traits associated with sexual behaviors in Mexican adolescents, their knowledge about contraception, and factors associated with pregnancy and utilization of contraceptives during first sexual intercourse. MATERIAL AND METHODS: Data from the Mexican National Health Survey 2000 (ENSA 2000) were analyzed. This study, conducted between September 1999 and March 2000, was a complex survey with a probabilistic, stratified, and cluster sampling design. The sample population included 15 241 adolescents 12 to 19 years of age. Data analysis included descriptive statistics and a chi-squared test for differences of proportions; also, logistic regression was used to obtain odds ratios. RESULTS: A total of 69.2% adolescents reported knowledge of at least one contraception method; 16.4% of subjects reported having had sexual intercourse. Males initiated sexual intercourse earlier than females and only 37% of all adolescents utilized contraceptives during their first sexual intercourse. Logistic regression analysis showed that being male, having a higher education, having knowledge of at least one contraception method, and having initiated sexual intercourse at an older age, were factors associated with utilization of a contraception method in their first sexual intercourse. A total of 55.7% of sexually active female adolescents had been pregnant. Pregnancy among adolescents was associated with low educational level, sexual activity at early age, and to have sometime lived in free union. CONCLUSIONS: The majority of sexually active adolescents did not use contraception during their first sexual intercourse, rendering them at risk of unwanted pregnancies.

Adolescent↗

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↗

PubMed2004

Application of lot quality assurance sampling for leprosy elimination monitoring--examination of some critical factors.

BACKGROUND: The concept of elimination of an infectious disease is different from eradication and in a way from control as well. In disease elimination programmes the desired reduced level of prevalence is set up as the target to be achieved in a practical time frame. Elimination can be considered in the context of national or regional levels. Prevalence levels depend on occurrence of new cases and thus could remain fluctuating. There are no ready pragmatic methods to monitor the progress of leprosy elimination programmes. We therefore tried to explore newer methods to answer these demands. With the lowering of prevalence of leprosy to the desired level of 1 case per 10000 population at the global level, the programme administrators' concern will be shifted to smaller areas e.g. national and sub-national levels. For monitoring this situation, we earlier observed that lot quality assurance sampling (LQAS), a quality control tool in industry was useful in the initially high endemic areas. However, critical factors such as geographical distribution of cases and adoption of cluster sampling design instead of simple random sampling design deserve attention before LQAS could generally be recommended. The present exercise was aimed at validating applicability of LQAS, and adopting these modifications for monitoring leprosy elimination in Tamil Nadu state, which was highly endemic for leprosy. METHODS: A representative sample of 64000 people drawn from eight districts of Tamil Nadu state, India, with maximum allowable number of 25 cases was considered, using LQAS methodology to test whether leprosy prevalence was at or below 7 per 10000 population. Expected number of cases for each district was obtained assuming Poisson distribution. Goodness of fit for the observed and expected cases (closeness of the expected number of cases to those observed) was tested through chi(2). Enhancing factor (design effect) for sample size was obtained by computing the intraclass correlation. RESULTS: The survey actually covered a population of 62157 individuals, of whom 56469 (90.8%) were examined. Ninety-six cases were detected and this number far exceeded the critical value of 25. The number of cases for each district and the number of cases in the entire surveyed area both followed Poisson distribution. The intraclass correlation coefficients were close to zero and the design effect was observed to be close to one. CONCLUSIONS: Based on the LQAS exercises leprosy prevalence in the state of Tamil Nadu in India was above 7 per 10000. LQAS method using clusters was validated for monitoring leprosy elimination in high endemic areas. Use of cluster sampling makes this method further useful as a rapid assessment procedure. This method needs to be tested for its applicability in moderate and low endemic areas, where the sample size may need increasing. It is further possible to consider LQAS as a monitoring tool for elimination programmes with respect to other disease conditions.

Adolescent↗

PubMed2005

Representation of individual gene expression in completely pooled mRNA samples.

Designing microarray experiments, scientists are often confronted with the question of pooling due to financial constraints, but discussion of the validity of pooling tends toward a sub-pooling recommendation. Since complete pooling protocols can be considered part of sub-pooling designs, gene expression data from three complete pooling experiments were analyzed. Data from complete pooled versus individual mRNA samples of rat brain tissue were compared to answer the question whether the pooled sample represents individual samples in small-sized experiments. Our analytic approach provided clear results concerning the Affymetrix MAS 5.0 signal and detection call parameters. Despite a strong similarity of arrays within experimental groups, the individual signals were evidently not appropriately represented in the pooled sample, with slightly more than half of all the genes considered. Our analysis reveals problems in cases of small complete pooling designs with less than six subjects pooled.

Animals↗

PubMed2007

Determination of organic additives in mortars by near-IR spectroscopy. A novel approach to designing a sample set with high-variability components.

Industrial mortars consist primarily of a mixture of cement and an aggregate plus a small amount of additives that are used to modify specific properties. Using too high or too low additive rates usually results in the loss of desirable properties in the end product. This entails carefully controlling the amounts of additives added to mortar in order to ensure correct dosing and/or adequate homogeneity in the final mixture. Near-IR (NIR) spectroscopy has proved effective for this purpose as it requires no sample pretreatment and affords expeditious analyses. The purpose of this work was to determine two organic additives (viz. Ad1 and Ad2) in mortars by using partial least squares regression multivariate calibration models constructed from NIR spectroscopic data. The additives are used to expedite setting and increase cohesion between particles in the mortar. In order to ensure that the sample set contained natural variability in the samples, we used a methodology based on experimental design to construct a representative set of samples. This novel design is based on a hexagonal antiprism that encompasses the concentration ranges spanned by the analytes and the variability inherent in each additive. The D-optimality criterion was used to obtain various combinations between Ad1 and Ad2 additive classes. The partial least squares calibration models thus constructed for each additive provided accurate predictions: the intercept and the slope of the plots of predicted values versus reference values for each additive were close to 0 and 1, respectively, and their confidence ranges included the respective value. The ensuing analytical methods were validated by using an external sample set.

Journal Article↗

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↗