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PubMedDate not supplied

An analysis of two-period crossover designs with carry-over effects.

The crossover design is a type of longitudinal study with subjects receiving different treatments in different time periods. When carry-over effects are absent, the usual crossover design is structured so that all the information about treatment effects is contained in the within-subject contrasts; standard analyses are based on these within-subject contrasts and ignore any between-subject information. With carry-over effects present these standard analyses can be very inefficient, especially for suboptimal designs. We describe alternative approaches based on methods for the analysis of longitudinal data.

Longitudinal Studies↗

PubMed2006

Statistical models in assessing fold change of gene expression in real-time RT-PCR experiments.

Real-time RT-PCR has been frequently used in quantitative research in molecular biology and bioinformatics. It provides remarkably useful technology to assess expression of genes. Although mathematical models for gene amplification process have been studied, statistical models and methods for data analysis in real-time RT-PCR have received little attention. In this paper, we briefly introduce current mathematical models, and study statistical models for real-time RT-PCR data. We propose a generalized estimation equations (GEE) model that properly reflects the structure of repeated data in RT-PCR experiments for both cross-sectional and longitudinal data. The GEE model takes the correlation between observations within the same subjects into consideration, and prevents from producing false positives or false negatives. We further demonstrate with a set of actual real-time RT-PCR data that different statistical models yield different estimations of fold change and confidence interval. The SAS program for data analysis using the GEE model is provided to facilitate easy computation for non-statistical professionals.

Adipose Tissue↗

PubMed2006

Do perceptions of friends' behaviors affect age at first sex? Evidence from Cebu, Philippines.

PURPOSE: To explore the effect of perceptions of friends' romantic and physical behaviors on adolescent sexual behavior and the relative influence of friends' sexual behaviors and maternal attitudes on age at first sex. METHODS: Using longitudinal data collected from 1943 adolescents who have never had sex at baseline in Cebu, Philippines, logistic regression assessed whether adolescents' perceptions of friends' sexual behaviors, measured at ages 14 to 16 years, were associated with age at first intercourse. Survival analysis assessed the hazard of having first intercourse by ages 17 to 19 years. Final models adjusted for sociodemographics, mothers'/caretakers' disapproval of premarital sex, living with a caretaker, and the adolescents' own behaviors at baseline. RESULTS: Boys and girls, who at ages 14 to 16 years, perceived that their friends had ever had boyfriends/girlfriends, dated, held hands, kissed, petted or had sex were significantly more likely to have experienced that behavior by ages 17 to 19 years. For each additional behavior an adolescent perceived his or her friends to have experienced, the hazard of having sex at a younger age increased by 1.15 (p = .02) among boys and 1.19 (p < .002) among girls, after adjusting for mothers'/caretakers' disapproval of premarital sex and other factors. CONCLUSIONS: These results demonstrate the important role of peers in light of competing influences in adolescents' lives. They provide support for asking at early ages what romantic and physical behaviors adolescents think their friends have experienced. Interventions can use such information to better prepare adolescents to make responsible and informed decisions about sexuality.

Adolescent↗

PubMed2005

[Patient and therapist satisfaction in outpatient psychotherapy].

Increasing attention is devoted to the patient perspective in mental health services research. Consequently, patient satisfaction has taken a prominent role and is frequently applied as an indicator of treatment quality and even outcome. However, the suitability of patient satisfaction for assessing quality of mental health services is doubtful, and there is a lack of longitudinal data especially for outpatient psychotherapy. By using data of the TRANS-OP study, we examine the course of patient satisfaction in outpatient psychotherapy over time and its predictors such as symptom change and duration of treatment. Furthermore, the relation between treatment satisfaction ratings of patients and therapists will be scrutinized. This study was conducted by the Center for Psychotherapy Research in Stuttgart (now Heidelberg) in cooperation with the Deutsche Krankenversicherung. N = 714 participants who applied for reimbursement of outpatient psychotherapy were consecutively recruited and asked to provide data at five (therapists two) measurement points during a two-year period. In line with previous evidence, high rates of patient satisfaction have been found, and also therapists appraised favorably the quality of their treatments. Course of patient satisfaction was found to be markedly stable over time. While some variables related to initial patient satisfaction, its course could hardly be predicted. Treatment outcome and session number moderately affected satisfaction at treatment termination. Concordance between patients' and therapists' satisfaction ratings was poor. Implications of these findings for research and practice are discussed.

Adult↗

PubMed1983

The vibrometer.

To evaluate the potential clinical value of the vibrometer in patients with peripheral nerve injury or compression, cutaneous vibratory thresholds were determined in 72 patients with 113 peripheral nerve problems. Testing with the vibrometer was quick, simple, and inexpensive. The test results, however, neither added new information nor improved information derived from evaluating sensibility with a tuning fork (vibratory stimulus) and a paper clip (innervation density). Following nerve repair, the vibratory threshold did not correlate with peripheral innervation density, and thus the vibrometer is not of prognostic value after neurorrhaphy. The vibrometer can provide quantitative sensory data useful in evaluating patients with bilateral peripheral nerve problems, thereby giving longitudinal data on nerve function in a research setting. This study confirmed the observation that the earliest clinical finding detectable with a tuning fork in peripheral nerve compression is a hypersensitive response to vibratory stimulation.

Carpal Tunnel Syndrome↗

PubMed2005

Preliminary evidence for persistent abnormalities in amygdala volumes in adolescents and young adults with bipolar disorder.

OBJECTIVES: Abnormalities in volumes of the amygdala have been reported previously in adolescents and adults with bipolar disorder (BD). Several studies have reported reduced volumes in adolescents with BD; however, both decreases and increases in volumes have been reported in adults with BD. Understanding of potential developmental contributions to these disturbances in morphology of the amygdala has been limited by the absence of longitudinal data in persons with BD. Here we use a within-subject longitudinal design to investigate whether amygdala volume abnormalities persist in adolescents and young adults with BD over a time interval of approximately 2 years. METHODS: Participants included 18 adolescents and young adults: 10 participants with BD I and 8 healthy comparison participants. Amygdala volumes were measured on high-resolution magnetic resonance imaging scans acquired twice for each subject over intervals of approximately 2 years. Amygdala volumes were the dependent measures in a mixed-model statistical analysis to compare amygdala volumes between groups over time while covarying for total brain volume. RESULTS: Amygdala volumes were significantly smaller in adolescents and young adults with BD compared with healthy participants (p = 0.018). The effect of time was not significant. CONCLUSIONS: Although the sample size is modest, this study provides preliminary evidence to support the presence of decreased amygdala volumes in adolescents and young adults with BD that persist during this developmental epoch.

Adolescent↗

PubMed2006

Hierarchical Bayesian methods for estimation of parameters in a longitudinal HIV dynamic system.

HIV dynamics studies have significantly contributed to the understanding of HIV infection and antiviral treatment strategies. But most studies are limited to short-term viral dynamics due to the difficulty of establishing a relationship of antiviral response with multiple treatment factors such as drug exposure and drug susceptibility during long-term treatment. In this article, a mechanism-based dynamic model is proposed for characterizing long-term viral dynamics with antiretroviral therapy, described by a set of nonlinear differential equations without closed-form solutions. In this model we directly incorporate drug concentration, adherence, and drug susceptibility into a function of treatment efficacy, defined as an inhibition rate of virus replication. We investigate a Bayesian approach under the framework of hierarchical Bayesian (mixed-effects) models for estimating unknown dynamic parameters. In particular, interest focuses on estimating individual dynamic parameters. The proposed methods not only help to alleviate the difficulty in parameter identifiability, but also flexibly deal with sparse and unbalanced longitudinal data from individual subjects. For illustration purposes, we present one simulation example to implement the proposed approach and apply the methodology to a data set from an AIDS clinical trial. The basic concept of the longitudinal HIV dynamic systems and the proposed methodologies are generally applicable to any other biomedical dynamic systems.

Antiretroviral Therapy, Highly Active↗

PubMed1999

Maternal mortality among the Kassena-Nankana of northern Ghana.

This report presents key findings from a maternal mortality study conducted in the Kassena-Nankana District of northern Ghana in 1997-98. Sibling history data collected in the course of this survey are analyzed together with longitudinal data from the Navrongo Demographic Surveillance System (NDSS). A comparison between mortality data from these two sources indicates that obtaining reasonably accurate estimates of age-specific death rates is possible by using the sisterhood method. Direct and indirect estimates from the maternal mortality study and the NDSS suggest a decline in the maternal mortality ratio for the Kassena-Nankana District from 800 to 600 maternal deaths per 100,000 live births over the past 14 years.

Adolescent↗

PubMed2003

Development of refractive error and strabismus in children with Down syndrome.

PURPOSE: To investigate the development of refractive errors and strabismus in a cohort of children with Down syndrome. METHOD: Data for 55 children with Down syndrome who are participating in this longitudinal study of visual development, first examined by us when aged less than 2 years and on at least two other occasions, were analyzed. Mohindra retinoscopy was used to measure refractive error. Ocular alignment was assessed using the Hirschberg test and, when possible, the cover test. RESULTS: Despite the high prevalence of large refractive errors in children with Down syndrome, longitudinal data showed that these are not always present in early infancy. Twenty-one (38%) of the children were emmetropic throughout the study. Of the 24 children with a significant refractive error at the outset, only 6 (25%) showed emmetropization. The others retained or increased their refractive errors. The remaining 10 children were emmetropic at the outset, but then had a significant refractive error develop. There is a high prevalence of strabismus in children with Down syndrome (29% of the total group), which cannot be attributed to the presence of hypermetropia or anisometropia. CONCLUSIONS: The retention or development of infantile refractive errors in many children with Down syndrome indicates a failure of emmetropization. All children were at risk of strabismus whatever the refractive error. The findings have implications for timing of screening programs.

Down Syndrome↗

PubMed2000

Outlet density and criminal violence in Norway, 1960-1995.

OBJECTIVE: To analyze the relationship between criminal violence and number of outlets for on-premise sales of alcohol. METHOD: The data comprise aggregate time series for Norway for the period 1960-95. Two crime indicators were used: (1) crimes of violence investigated by the police per 100,000 inhabitants aged 15 years and above (15+) and (2) convictions for criminal violence per 100,000 inhabitants (15+). Outlet density was measured as number of public drinking places per 10,000 inhabitants (15+). RESULTS: On the basis of autoregressive integrated moving average (ARIMA) analyses of differenced data, a positive and statistically significant relationship (p = .03) was found between outlet density and crimes of violence investigated by the police. This replicates the findings that are reported from cross-sectional studies. The relationship was also positive, but of borderline significance (p = .06), when convictions for criminal violence were used as outcome. CONCLUSIONS: The findings suggest that increases in the number of public drinking places are associated with increases in criminal violence. Although other kinds of data support the findings, these are the first to be based on longitudinal data. To test for robustness and cultural specificity it, therefore, seems warranted to replicate the study on data from other drinking cultures.

Alcoholic Beverages↗

PubMedDate not supplied

Perceived vulnerability in children and adolescents.

Some lines of development of research on children's perceptions of vulnerability to a variety of health problems are discussed, with special reference to cross-sectional data from samples of American and French-speaking Canadian youngsters, and longitudinal data from two American samples. Perceived vulnerability demonstrated some developmental changes but these were not consistent or appreciable. The measurement of perceived vulnerability has been expanded to include serious health problems during adult years. Females showed consistently higher levels than males. Perceived vulnerability was negatively related to a variety of preventive health behaviors and to self-concept, and positively related to anxiety. The data suggest that perceived vulnerability has systemic qualities as well as important affective properties, and that these must be considered by health educators and other health professionals to assure greater effectiveness for their programs and interventions.

Adolescent↗

PubMed1994

Outlier reduction by an option-3 measurement scheme.

Detecting changes in longitudinal data is important in medical research. However, the existence of measurement outliers can cause an unexpected increase in the false alarm rate in claiming changes. To reduce the outliers, a new method has been developed. In this scheme, two measures are initially taken and, if they are closer than a specified threshold, the average of the two is considered to be the estimate of the true mean; otherwise a third measurement is taken, and the mean of the closest pair is considered to be the estimate. It is shown that this method has considerable sample size advantage over naive repeated measurements. Moreover, this scheme is robust for outlier error distribution. Evidence on outlier removal in dental attachment probing is used as an example.

Analysis of Variance↗

PubMed1996

Intent-to-treat analysis for longitudinal studies with drop-outs.

We consider intent-to-treat (IT) analysis of clinical trials involving longitudinal data subject to drop-out. Common methods, such as Last Observation Carried Forward imputation or incomplete-data methods based on models that assume random dropout, have serious drawbacks in the IT setting. We propose a method that involves multiple imputation of the missing values following drop-out based on an "as treated" model, using actual dose after drop-out if this is known, or imputed doses that incorporate a variety of plausible alternative assumptions if unknown. The multiply-imputed data sets are then analyzed using IT methods, were subjects are classified by randomization group rather than by the dose actually received. Results from the multiply-imputed data sets are combined using the methods of Rubin (1987, Multiple Imputation for Nonresponse in Surveys). A novel feature of the proposed method is that the models for imputation differ from the model used for the analysis of the filled-in data. The method is applied to data on a clinical trial for Tacrine in the treatment of Alzheimer's disease.

Alzheimer Disease↗

PubMedDate not supplied

The consistency of self-reported initiation of sexual activity.

In an analysis of the consistency of self-reported age at first intercourse using longitudinal data from the National Youth Survey, 28-32% of adolescents reported an age at first intercourse inconsistent with the information they provided up to seven years later as adults. Overall, while females were the most likely to offer consistent responses (70%), while black males were the least likely to do so (27%). Multivariate analyses indicated that in addition to race and gender, some social and economic factors were significantly associated with inconsistent reporting. For example, those who lived in a two-parent household were less likely than those from a one-parent family to report an earlier age at first intercourse as adolescents than they reported as adults. After controlling for these inconsistencies, overall predictors of adolescent sexual behavior remained unchanged. Although the analysis could not determine which time period reflected more accurate data, it does suggest limitations to using self-reported data to construct estimates of adolescent sexual activity and to evaluate programs designed to alter adolescent sexual behavior.

Adolescent↗

PubMed2004

Five-year obesity incidence in the transition period between adolescence and adulthood: the National Longitudinal Study of Adolescent Health.

BACKGROUND: No nationally representative longitudinal data have been analyzed to evaluate the incidence of obesity in the transition between adolescence and adulthood. OBJECTIVE: The objective was to examine dynamic patterns of change in obesity among white, black, Hispanic, and Asian US teens as they transitioned to young adulthood. DESIGN: We used nationally representative, longitudinally measured height and weight data collected from US adolescents enrolled in wave II (1996; ages 13-20 y) and wave III (2001; 19-26 y) of the National Longitudinal Study of Adolescent Health (n = 9795). Obesity incidence was defined on the basis of International Obesity Task Force (IOTF) cutoffs (wave II), which link childhood body mass index (BMI) centiles to adult cutoffs (BMI > or = 30; wave III), for comparability between adolescence and adulthood. In addition, the more commonly used cutoff of BMI > or = 95th percentile for age- and sex-specific cutoffs from the 2000 Centers for Disease Control and Prevention growth charts for adolescents (wave II) were compared with adult cutoffs (BMI > or = 30; wave III). RESULTS: On the basis of the IOTF cutoffs, obesity incidence over the 5-y study period was 12.7%; 9.4% of the population remained obese and 1.6% shifted from obese to nonobese. Obesity incidence was especially high in non-Hispanic black (18.4%) females relative to white females. The prevalence of obesity increased from 10.9% in wave II to 22.1% in wave III, and extreme obesity was 4.3% at wave III on the basis of a BMI > or = 40. CONCLUSIONS: During a 5-y transitional period between adolescence and young adulthood, the proportion of adolescents becoming and remaining obese into adulthood was very high. This upward trend is likely to continue. Effective preventive and treatment efforts are critically needed.

Adolescent↗

PubMed2003

Personality over time: Methodological approaches to the study of short-term and long-term development and change.

Although theories of personality emphasize the integrative, enduring, and dynamic nature of personality, the current modal research design in personality ignores the dimension of time. We consider a variety of recent methods of longitudinal data analysis to examine both short-term and long-term development and change in personality, including mean-level analyses both across and within individuals across time, variance structures across time, and cycles and dynamic models across time. These different longitudinal analyses can address classic as well as new questions in the study of personality and its development. We discuss the linkages among different longitudinal analyses, measurement issues in temporal data, the spacing of assessments, and the levels of generalization and potential insights afforded by different longitudinal analyses.

Humans↗

PubMed2001

Linear equality constraints in the general linear mixed model.

Scientists may wish to analyze correlated outcome data with constraints among the responses. For example, piecewise linear regression in a longitudinal data analysis can require use of a general linear mixed model combined with linear parameter constraints. Although well developed for standard univariate models, there are no general results that allow a data analyst to specify a mixed model equation in conjunction with a set of constraints on the parameters. We resolve the difficulty by precisely describing conditions that allow specifying linear parameter constraints that insure the validity of estimates and tests in a general linear mixed model. The recommended approach requires only straightforward and noniterative calculations to implement. We illustrate the convenience and advantages of the methods with a comparison of cognitive developmental patterns in a study of individuals from infancy to early adulthood for children from low-income families.

Adolescent↗

PubMed2006

Joint models for multivariate longitudinal and multivariate survival data.

Joint modeling of longitudinal and survival data is becoming increasingly essential in most cancer and AIDS clinical trials. We propose a likelihood approach to extend both longitudinal and survival components to be multidimensional. A multivariate mixed effects model is presented to explicitly capture two different sources of dependence among longitudinal measures over time as well as dependence between different variables. For the survival component of the joint model, we introduce a shared frailty, which is assumed to have a positive stable distribution, to induce correlation between failure times. The proposed marginal univariate survival model, which accommodates both zero and nonzero cure fractions for the time to event, is then applied to each marginal survival function. The proposed multivariate survival model has a proportional hazards structure for the population hazard, conditionally as well as marginally, when the baseline covariates are specified through a specific mechanism. In addition, the model is capable of dealing with survival functions with different cure rate structures. The methodology is specifically applied to the International Breast Cancer Study Group (IBCSG) trial to investigate the relationship between quality of life, disease-free survival, and overall survival.

Acquired Immunodeficiency Syndrome↗