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PubMed1998

Sample size calculations based on slopes and other summary statistics.

Sample size calculations based on two-sample comparisons of slopes have been reported by many. This paper extends such discussions to include summary statistics other than slopes, such as post-baseline means, change scores, and final observations. Specifically, sample size formulas for analyses based on a broad class of summary statistics are presented, with modifications proposed to allow for missing data caused by staggered entry and random dropouts. The formulas developed are used to illustrate how required sample size is affected by summary statistic choice, variance parameters, the type of treatment difference of interest, and the manner in which incomplete observations are used in the analysis. An example based on longitudinal data from the Muscatine Study is presented.

Adolescent↗

PubMed2003

An empty toolbox? Changes in health plans' approaches for managing costs and care.

OBJECTIVE: To examine how health plans have changed their approaches for managing costs and utilization in the wake of the recent backlash against managed care. DATA SOURCES/STUDY SETTING: Semistructured interviews with health plan executives, employers, providers, and other health care decision makers in 12 metropolitan areas that were randomly selected to be nationally representative of communities with more than 200,000 residents. Longitudinal data were collected as part of the Community Tracking Study during three rounds of site visits in 1996-1997, 1998-1999, and 2000-2001. STUDY DESIGN: Interviews probed about changes in the design and operation of health insurance products--including provider contracting and network development, benefit packages, and utilization management processes--and about the rationale and perceived impact of these changes. DATA COLLECTION/EXTRACTION METHODS: Data from more than 850 interviews were coded, extracted, and analyzed using computerized text analysis software. PRINCIPAL FINDINGS: Health plans have begun to scale back or abandon their use of selected managed care tools in most communities, with selective contracting and risk contracting practices fading most rapidly and completely. In turn, plans increasingly have sought cost savings by shifting costs to consumers. Some plans have begun to experiment with new provider networks, payment systems, and referral practices designed to lower costs and improve service delivery. CONCLUSIONS: These changes promise to lighten administrative and financial burdens for physicians and hospitals, but they also threaten to increase consumers' financial burdens.

Contract Services↗

PubMed1996

A prospective 4-year follow-up study of attention-deficit hyperactivity and related disorders.

BACKGROUND: Previous cross-sectional data showed that children and adolescents with attention-deficit hyperactivity disorder (ADHD) are at increased risk of comorbid conduct, mood, and anxiety disorders as well as impairments in cognitive, social, family, and school functioning. However, longitudinal data were needed to confirm these initial impressions. METHODS: Using DSM-III-R structured diagnostic interviews and raters blinded as to diagnosis, we reexamined psychiatric diagnoses at 1- and 4-year follow-ups in children with ADHD and controls. In addition, subjects were evaluated for cognitive, achievement, social, school and family functioning. RESULTS: Analyses of follow-up findings revealed significant differences between children with ADHD and controls in rates of behavioral, mood, and anxiety disorders, with these disorders increasing markedly from baseline to follow-up assessments. In addition, children with ADHD had significantly more impaired cognitive, family, school, and psychosocial functioning than did controls. Baseline diagnosis of conduct disorder predicted major depression and bipolar disorder at follow-up, and anxiety disorders at baseline predicted anxiety disorders at follow-up. CONCLUSION: These results confirm and extend previous retrospective results indicating that children with ADHD are at high risk of developing a wide range of impairments affecting multiple domains of psychopathology such as cognition, interpersonal, school, and family functioning. These findings provide further support for the value of considering psychiatric comorbidity in both clinical assessment and research protocols involving children with ADHD.

Achievement↗

PubMed1995

Determinants of postnatal weight in infant rhesus monkeys: implications for the study of interindividual differences in neonatal growth.

This paper provides an analysis of infant body weights obtained from a sample of 38 rhesus monkey infants (Macaca mulatta) aged 29-165 days, i.e., animals still nutritionally dependent on their mothers. We examine the data on neonatal weights in relation to a number of factors, most notably, the sex of the infants, and the age and adiposity of their mothers. The infant body weights represent cross-sectional rather than longitudinal data; because they were mostly free-ranging animals, the infants were weighted just once each. Nevertheless, the results of our analysis strongly suggest that early postnatal growth in free-ranging rhesus is dependent on both maternal fatness and age. They also suggest that, although male infants are generally heavier than like-aged female infants, they do not grow any faster during the early postnatal period. Here, we speculate that the associations between infant size and both maternal age and adiposity are the result of between-mother differences in lactational output.

Adipose Tissue↗

PubMed2001

Analytic approaches for assessing long-term treatment effects. Examples of empirical applications and findings.

Analytic approaches, including the structural equation model (autoregressive panel model), hierarchical linear model, latent growth curve model, survival/event history analysis, latent transition model, and time-series analysis (interrupted time series, multivariate time-series analysis) are discussed for their applicability to data of different structures and their utility in evaluating temporal effects of treatment. Methods are illustrated by presenting applications of the various approaches in previous studies examining temporal patterns of treatment effects. Recent advancements in these longitudinal modeling approaches and the accompanying computer software development offer tremendous flexibility in examining long-term treatment effects through longitudinal data with varying numbers and intervals of assessment and types of measures. A multimethod assessment will contribute to a more complete understanding of the complex phenomena of the long-term courses of substance use and its treatment.

Alcoholism↗

PubMed2003

Nonparametric longitudinal allele-sharing model.

Basically no methods are available for the analysis of quantitative traits in longitudinal genetic epidemiological studies. We introduce a nonparametric factorial design for longitudinal data on independent sib pairs, modelling the phenotypic quadratic differences as the dependent variable. Factors are the number of alleles shared identically by descent (IBD) and the age categories at which the dependent variable is measured, allowing for dependence due to age. To identify a linked marker a rank statistic tests the influence of IBD group on phenotypic quadratic differences. No assumptions are made on normality or variances of the dependent variable. We apply our method to 71 sib pairs from the Framingham Heart Study data provided at the Genetic Analysis Workshop 13. For all 15 available markers on chromosome 17 we analyzed the influence on systolic blood pressure. In addition, different selection strategies to sample from the whole data are discussed.

Adult↗

PubMed2000

NCCN outcomes research database: data collection via the Internet.

Since 1997, the National Comprehensive Cancer Network (NCCN) outcomes database has successfully collected and reported patterns of care and outcomes data in the oncology setting. Breast cancer was the first cancer site chosen for this outcomes study, with non-Hodgkin's lymphoma targeted as the next site. Quarterly reports of the data are compiled and distributed to all participating centers. The results for each institution are given to the Principal Investigator at the respective institution. At present, the NCCN database contains more than 100,000 records of longitudinal data on more than 5,000 women with breast cancer. Extending data collection to the community setting is a goal.

Breast Neoplasms↗

PubMedDate not supplied

Teenage pregnancy and child health in the urban Sahel.

Longitudinal data for more than 20,000 live births in the cities of Bamako (Mali) and Bobo-Dioulasso (Burkina Faso) are used to study the effects of young maternal age (less than 18 years and 18-19) on birth weight, child health care and feeding behavior, and child mortality, after controlling for other socioeconomic and demographic factors. Teenage pregnancies are associated with significantly worse prenatal health care and vaccination behavior, lower birth weights, earlier weaning, and, especially during the second year of life, higher mortality. A proxy for mother's school enrollment at the time of pregnancy is strongly linked to worse prenatal health behavior and weakly associated with other poor behaviors and health outcomes. Overall, the results highlight the importance of behavioral factors relative to strictly biological factors for explaining child health differentials.

Adolescent↗

PubMed1999

Effects of comorbidity on health-related quality-of-life scores: an analysis of clinical trial data.

Coexisting diseases may have unforeseen yet clinically significant effects on patients' well-being. Both generic and disease-specific measures are frequently used to assess health-related quality of life (QOL). The present study assessed the effects of comorbidity on the results of QOL measures through an analysis of longitudinal data from 3 double-masked, randomized, placebo-controlled clinical trials dealing with heartburn, asthma, and ulcer. Patients were assigned to subgroups by comorbidity status: those with no comorbid diseases and those whose principal disease was heartburn, asthma, or ulcer and whose comorbid condition was chronic obstructive pulmonary disease, asthma, or chronic bronchitis; hypertension; migraine, coronary artery disease, or varicose veins; chronic gastrointestinal conditions; arthritis or back pain; diabetes; or depression. Multivariate analysis of covariance was used to test the study hypotheses. The study results suggest that comorbid conditions significantly and extensively affect patients' scores on generic QOL measures and estimation of treatment effect, whereas their influence on disease-specific QOL scores and estimation of treatment effect is considerably smaller. Further, the most important comorbidities in the 3 trial populations were arthritis or back pain and depression, which respectively accounted for 17% and 5% of the patient population. These findings have significant practical implications for the estimation of true treatment effects, control of comorbidity effects, and design of QOL trials.

Asthma↗

PubMed2000

On the application of queuing theory for analysis of twin data.

A mathematical model based on queuing theory is used to study the dynamics of environmental influence on twin pairs. The model takes into consideration genetic factors and effects of nonshared environment. Histograms are exploited as base analysed characteristics, with the method of minimum chi-square yielding estimated characteristics. The proposed technique was applied to analysis of longitudinal data for MZ and DZ twins. It was shown that the same environment impact may yield different contributions to final variances of the IQ parameters under investigation. Magnitudes of these contributions depend on the genetic factor represented by distributions of an analysed parameter at the point of birth.

Adolescent↗

PubMed1978

A computer program for multivariate ratio analysis (MISCAT).

Analysts must deal frequently with missing data in multivariate analysis. In such cases, estimating the covariance maxtrix V of the dependent variables usually involves initial estimation and iterative adjustment of imputed missing data values, and/or smoothing of an estimate V which is not necessarily positive semi-definite. This paper presents an alternative procedure for computing estimates of relevant multivariate parameters in situations where missing data occur at random and with small probability. MISCAT is a computer program which computes multivariate ratio estimates of the means and a corresponding positive semi-definite estimate of the covariance matrix. It is an extension of GENCAT, which is a program for the generalizaed least squares analysis of categorical data. Thus, one advantage of dealing with missing data in this manner is that variation among the ratio estimates may be conveniently analyzed within MISCAT using asymptotic regression methodology, provided that sample sizes are sufficiently large. An example is given to illustrate such analysis for longitudinal data from a multicenter clinical trial.

Computers↗

PubMed2005

Can glatiramer acetate reduce brain atrophy development in multiple sclerosis?

The assessment of brain volume changes on serial magnetic resonance imaging (MRI) scans can provide an objective measure of progressive atrophy reflecting the neurodegenerative aspects of multiple sclerosis (MS) pathology. The present article reviews the results of studies assessing the effect of glatiramer acetate (GA) treatment in preventing MS-related, MRI-measurable brain volume decrease. Whilst data from the extended, open-label follow-up of the US trial seem to indicate that long-term treatment with GA might prevent the loss of brain parenchyma in relapsing-remitting MS patients, longitudinal data from the European/Canadian MRI trial suggest that, over a short-term period of treatment, GA does not have a clear-cut impact on the decrease of brain volume. The effect of GA on MS-related brain atrophy might, therefore, be delayed and dissociated in time from those exerted on other clinical and MRI measures of disease activity. However, the modest magnitude of this effect makes it difficult to evaluate its impact on the actual disease progression. Further studies of adequate duration are now required to address this issue, as well as to confirm the sustained efficacy of GA treatment over long periods of follow-up.

Animals↗

PubMed2003

Longitudinal phylogenetic tree of within-host viral evolution from noncontemporaneous samples: a distance-based sequential-linking method.

A new method for reconstructing phylogenetic relationships of within-host (patient) viral evolution from noncontemporaneous samples is presented. This method has two important features: noncontemporaneous viral samples can be dealt with by a simple computing algorithm, and both neutral and adaptive evolution patterns occurring during the process of viral evolution can be estimated. In our previous study, we proposed a preliminary formulation of this algorithm that was based on the maximum likelihood method. However, that preliminary formulation was difficult to use because the calculation of the likelihood required an extremely large amount of time and the number of possible tree topologies increased exponentially according to the increase in the number of viral variants. In this paper, we propose another new algorithm, referred to as a distance-based sequential-linking algorithm, in which the neighbor-joining method is employed for reconstruction of the longitudinal phylogenetic tree from serial viral samples. This algorithm is applied to a longitudinal data set of the env gene (V3 region) of human immunodeficiency virus type 1 (HIV-1) obtained over 7 years after the infection of a single patient. The results suggest that this method can successfully reconstruct a longitudinal phylogenetic tree from noncontemporaneous viral samples within a reasonable calculation time. This revised method proved to be a useful tool for estimating the dynamic process of within-host viral evolution.

Evolution, Molecular↗

PubMed1997

An epidemiologic review of dietary intake studies among American Indians and Alaska Natives: implications for heart disease and cancer risk.

PURPOSE: Dietary factors play an important role in the occurrence of heart disease and cancer. While American Indians and Alaska Natives (AIANs) have unique heart disease and cancer mortality profiles, little is known about the effect of diet on heart disease and cancer risk in these populations. This paper reviews existing nutritional intake data from adult AIANs, and considers the potential impact of diet on heart disease and cancer in these communities. METHODS: A review of the literature was conducted using the Medline database system and other reference materials. Studies documenting nutrient intakes only were included in this review. Studies were limited to those among healthy, non-pregnant adults. RESULTS: A total of twelve reports from 1959 to 1996 were found. Sample sizes for the studies ranged from 20 to 575 subjects. Most studies were done among women, and a variety of nutritional assessment techniques (24 hour recall, food frequency questionnaire, multiple-day food record) were used. Most studies also had limited nutrient intake data, especially for dietary fiber and vitamin E. The majority of studies reported moderately high intakes of fat and saturated fat, and low intakes of polyunsaturated fat and fiber. CONCLUSIONS: Based on the limited data, diet may play an important role in the heterogeneity of heart disease and cancer mortality in AIAN communities. More research is needed to assess the impact of diet on heart disease and cancer risk, including more longitudinal data, and data to assess the validity and reliability of traditional methods of dietary assessment.

Adult↗

PubMed2001

Sleep disturbance in orofacial pain patients: pain-related or emotional distress?

Associations between pain, depression, and sleep disturbance have been documented in several chronic pain patient samples. The current study assessed the prevalence and magnitude of sleep disturbance in a sample of 128 orofacial pain patients referred for clinical evaluation and tested linkages between sleep, depression, anxiety, and pain using cross-sectional and longitudinal data. Seventy-seven percent of the patients reported reduced sleep quantity since pain onset. In cross-sectional analyses, reduced sleep quantity was associated with depression and pain. Reduced sleep quality was associated with negative affect. Longitudinally, initial depression and pain predicted sleep at time two and initial pain predicted negative affect. Sleep did not predict pain. Results support the hypothesis that pain, rather than sleep disturbance, increases negative affect across time, whereas negative affect is more a cause of concurrent reduced sleep quality than is pain. The results highlight the importance of assessing for sleep disturbance in orofacial pain patients.

Adolescent↗

PubMed1994

Time-varying covariates in models of human mortality and aging: multidimensional generalizations of the Gompertz.

Models of mortality and aging depend on assumptions about physiological change even if they are not made explicit. Standard models, like the Gompertz, often fail to describe mortality at extreme ages, suggesting a need for biologically more detailed and flexible models. One solution is to model the interaction of time-varying covariates with mortality to better describe the age dependence of mortality, test hypotheses about the relation of physiological change and mortality, and use longitudinal data to generalize assumptions about physiological change. This model is applied to (a) a 34-year follow-up of risk factors and mortality and (b) a 9.5-year follow-up of function and mortality from longitudinal surveys of the U.S. elderly population.

Adult↗

PubMed2006

Intrastromal corneal ring implantation (360 degrees ring) for myopia: a 5-year follow-up.

PURPOSE: Stability of correction is a major factor for successful refractive surgery. Intrastromal corneal rings were placed for the correction of low to moderate myopia beginning 10 years ago. The purpose of this study was to evaluate findings in patients 5 years after ring placement and to compare these findings with 1-year results to assess the stability of effect. METHODS: Seventy-two patients (113 eyes) with myopia (range, -0.75 to -4.50 diopters [D]) were treated with the placement of intrastromal corneal rings (360 degrees ), in a multicenter clinical trial between 1993 and 1994 under U.S. Food and Drug Administration phase II and phase III clinical trials. Six insert sizes were evaluated: 0.21, 0.25, 0.30, 0.35, 0.40, and 0.45 mm. The long-term results that were evaluated were uncorrected visual acuity, best spectacle-corrected visual acuity, cycloplegic refraction spherical equivalent at 5 years and proximity of this value to the target refraction determined preoperatively, induced manifest refraction cylinder, and slitlamp findings. RESULTS: Data at the 5-year follow-up showed that uncorrected visual acuity was 20/40 or better in 83% of eyes and 20/20 or better in 64% of eyes, compared to 88% and 43%, respectively, at the first year of follow-up. Only 7% of eyes lost two or more lines of preoperative best spectacle-corrected visual acuity at 5 years, compared to 11% at the first year. No eyes at the 1- or 5-year follow-up had a best spectacle-corrected visual acuity worse than 20/40. Cycloplegic refraction spherical equivalent was within 1.00 D of target refraction in 68% of eyes at 5 years, compared to 71% at the 1-year follow-up. Induced manifest refraction cylinder greater than 1.00 D was reduced to 5% of eyes in the fifth year, compared to 12% of eyes at year 1. CONCLUSIONS: Intrastromal corneal rings are a safe, effective, and stable method of correcting mild to moderate myopia, and most patients continue to be satisfied with the results after 5 years. There was no clinically significant change in refractive effect and the central corneas remained clear at the 5-year follow-up in all eyes studied. Further data are currently being collected for 10-year follow-up periods. It is hypothesized that the use of intrastromal corneal rings as a refractive option for mild to moderate myopia will be supported by these additional longitudinal data.

Corneal Stroma↗

PubMed2002

Influence of the beta2-adrenergic receptor Arg16Gly polymorphism on longitudinal changes in obesity from childhood through young adulthood in a biracial cohort: the Bogalusa Heart Study.

BACKGROUND: The beta2-adrenergic receptor (ADRB2) plays a major role in regulating energy expenditure by stimulating lipid metabolism in human adipose tissue. Polymorphisms in the ADRB2 gene have been associated with obesity and various weight-related traits in cross-sectional studies of adults, but little is known about the effects of the ADRB2 gene on childhood obesity or the propensity to gain weight over time. OBJECTIVE: To assess the effects of a polymorphism in codon 16 (Arg16-->Gly) of the ADRB2 gene, which has been associated with a decrease in beta2-receptor density and efficiency, on longitudinal changes in obesity from childhood to young adulthood in a biracial cohort. DESIGN: Seven cross-sectional screenings of children and five cross-sectional screenings of young adults who were previously examined as children produced longitudinal data from childhood to young adulthood. METHODS: Height, weight and subscapular and triceps skinfolds were measured by trained examiners following identical protocols over the course of the study. Gender- and age-stratified analyses using random coefficients models were used to examine longitudinal genetic effects on obesity in 1151 African-American and Caucasian males and females who attended an average of six examinations over a 24 y period from childhood to young adulthood. RESULTS: Age-stratified analyses showed no clear genetic relationships with changes in obesity measures over time in females, but an age-dependent association was observed in males, where the relationship between the Arg16Gly polymorphism and obesity became stronger with age. In males who were 4-9 y of age at the beginning of the study in 1973, body mass index (BMI) was 4% higher in Gly/Gly and Arg/Gly males compared to those with Arg/Arg by 26 y of age. Subscapular skinfold measurements in Gly/Gly males became significantly different from Arg/Arg males (20% higher) by age 20. In the oldest male cohort (10-14 y of age in 1973), BMI increased at a significantly greater rate (0.4%/y) in males carrying the Gly16 form of the receptor relative to Arg/Arg males. BMI was significantly different between homozygous genotypes by approximately 26 y of age, and reached 8% higher in Gly/Gly males by age 32. Subscapular skinfolds also increased at a significantly greater rate (2%/y) in Gly/Gly males compared to Arg/Arg males, becoming significantly different (27%) by approximately 22 y of age and reaching a maximum difference of 50% by age 32. CONCLUSIONS: Our data suggest that the beta2-adrenergic receptor is associated with the propensity to gain weight from childhood to young adulthood in males. An increased understanding of genetic influences on the development of obesity may improve the effectiveness of interventions designed to reduce excess body weight and help define the role of genetic factors in diabetes and cardiovascular disease.

Alleles↗