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PubMedDate not supplied

Caries occurrence in a fluoridated and a nonfluoridated town in Finland: a retrospective study using longitudinal data from public dental records.

The tap water of Kuopio, Finland, was fluoridated from 1959 to 1992. In the first decade of fluoridation, children in Kuopio had lower DMF values than children in Jyväskylä, a nearby low-fluoride town, but later differences between the towns have been small and inconsistent. The present study aimed to gain further insight into caries occurrence in Kuopio and Jyväskylä using longitudinal tooth-specific data from public health records on cohorts born in 1970/71 and 1980/81 (total n = 1,503). Survival analyses were used to summarize the tooth-specific times elapsed between eruption and the first filling (used as a proxy for dental caries). Generally, the first filling was placed sooner after eruption in the 1970/71 cohort than in the 1980/81 cohort. The curves for the two towns were virtually identical except for the first molars of the 1970/71 cohort, for which the percentage of filled first molars was consistently lower in Jyväskylä than in Kuopio. This study indicates that, among children and adolescents whose permanent teeth erupted in the mid-1970s or thereafter, even a longitudinal approach did not reveal a lower caries occurrence in the fluoridated than in the low-fluoride reference community. The main reason for the modest effect of water fluoridation in Finnish circumstances is probably the widespread use of other measures for caries prevention. The children have been exposed to such intense efforts to increase tooth resistance that the effect of water fluoridation does not show up any more. The results must not be extrapolated to countries with less intensive preventive dental care.

Adolescent↗
PubMed1989

The robustness of the terminal decline phenomenon: longitudinal data from the Digit-Span Memory Test.

The robustness of terminal decline was studied by analyzing longitudinal results from the simple Digit-Span Memory Test. The subjects participated in the Gothenburg longitudinal study and were first examined at the age of 70. Analyses were conducted for different groups according to subsequent survival. The findings provided support for the robustness of the terminal decline phenomenon.

Aged↗
PubMed1991

Radionuclide esophageal transit studies in progressive systemic sclerosis: an analysis of longitudinal data.

Nineteen patients with progressive systemic sclerosis (PSS) were studied by radionuclide esophageal transit (ET) and followed longitudinally for 3 to 5 years. Results were expressed as percent retention at 20 s and 10 min. There was gradual deterioration of ET at both 20 s and 10 min. When results were grouped into quartiles, deterioration occurred in 58.5% of followup studies in patients who initially had potential to deteriorate regarding 20 s retention and in 48% of similar patients regarding 10 min retention.

Adult↗
PubMed2002

Bias adjustment in analysing longitudinal data with informative missingness.

The recent biostatistical literature contains a number of methods for handling the bias caused by 'informative censoring', which refers to drop-out from a longitudinal study after a number of visits scheduled at predetermined intervals. The same or related methods can be extended to situations where the missing pattern is intermittent. The pattern of missingness is often assumed to be related to the outcome through random effects which represent unmeasured individual characteristics such as health awareness. To date there is only limited experience with applying the methods for informative censoring in practice, mostly because of complicated modelling and difficult computations. In this paper, we propose an estimation method based on grouping the data. The proposed estimator is asymptotically unbiased in various situations under informative missingness. Several existing methods are reviewed and compared in simulation studies. We apply the methods to data from the Wisconsin Diabetes Registry Project, a longitudinal study tracking glycaemic control and acute and chronic complications from the diagnosis of type I diabetes.

Bias↗
PubMed2005

Effects of variance-function misspecification in analysis of longitudinal data.

The approach of generalized estimating equations (GEE) is based on the framework of generalized linear models but allows for specification of a working matrix for modeling within-subject correlations. The variance is often assumed to be a known function of the mean. This article investigates the impacts of misspecifying the variance function on estimators of the mean parameters for quantitative responses. Our numerical studies indicate that (1) correct specification of the variance function can improve the estimation efficiency even if the correlation structure is misspecified; (2) misspecification of the variance function impacts much more on estimators for within-cluster covariates than for cluster-level covariates; and (3) if the variance function is misspecified, correct choice of the correlation structure may not necessarily improve estimation efficiency. We illustrate impacts of different variance functions using a real data set from cow growth.

Animals↗
PubMed2005

Reference ranges for umbilical vein blood flow in the second half of pregnancy based on longitudinal data.

OBJECTIVES: To construct new reference ranges for serial measurements of umbilical vein (UV) blood flow. METHODS: Prospective longitudinal study of blood flow velocities and diameter of the UV measured at four-weekly intervals during 19 to 42 weeks' gestation in 130 low-risk singleton pregnancies. Regression models and multilevel modeling were used to construct the reference ranges. RESULTS: On the basis of 511 sets of longitudinal observations, we established new reference percentiles of UV diameter, blood flow velocities, volume flow, and blood flow normalized for fetal weight and abdominal circumference. They reflected some of the developmental patterns of previous cross-sectional studies, but with important differences, particularly near term. The UV blood flow showed a continuous increase until term, whereas the flow normalized per unit fetal weight, a corresponding reduction. Calculating the blood flow on the basis of intensity-weighted mean velocity or 0.5 of the maximum velocity gave almost interchangeable results for most fetuses. CONCLUSION: New reference ranges for UV blood flow based on longitudinal observations appear slightly different from cross-sectional studies, and should be more appropriate for serial evaluation of fetal circulation.

Adolescent↗
PubMed1998

Covariate measurement error and the estimation of random effect parameters in a mixed model for longitudinal data.

We explore the effects of measurement error in a time-varying covariate for a mixed model applied to a longitudinal study of plasma levels and dietary intake of beta-carotene. We derive a simple expression for the bias of large sample estimates of the variance of random effects in a longitudinal model for plasma levels when dietary intake is treated as a time-varying covariate subject to measurement error. In general, estimates for these variances made without consideration of measurement error are biased positively, unlike estimates for the slope coefficients which tend to be 'attenuated'. If we can assume that the residuals from a longitudinal fit for the time-varying covariate behave like measurement errors, we can estimate the original parameters without the need for additional validation or reliability studies. We propose a method to test this assumption and show that the assumption is reasonable for the example data. We then use a likelihood-based method of estimation that involves a simple extension of existing methods for fitting mixed models. Simulations illustrate the properties estimators.

Analysis of Variance↗
PubMed1995

Health effects of anticipation of job change and non-employment: longitudinal data from the Whitehall II study.

OBJECTIVE: To assess the effect of anticipating job change or non-employment on self reported health status in a group of middle aged male and female white collar civil servants. DESIGN: Longitudinal cohort study (Whitehall II study). Questionnaire data on self reported health status and health behaviour were obtained at initial screening and four years later, during the period when employees of the department facing privatisation were anticipating job change or job loss. SETTING: London based office staff in 20 civil service departments. SUBJECTS: 666 members of one department threatened with early privatisation were compared with members of the 19 other departments. MAIN OUTCOME MEASURES: Self reported health status measures and health related behaviours, before and during anticipation of privatisation. RESULTS: In comparison to the remainder of the cohort, the profile of health related behaviours of cohort members who faced privatisation was more favourable, both before and during anticipation of privatisation. There were no significant differences in the changes in health behaviours between cohort members moving into a period of job insecurity and the remainder of the cohort. Self reported health status, however, tended to deteriorate among employees anticipating privatisation when compared with that of the rest of the cohort. CONCLUSIONS: The application of a longitudinal design, allowing the same individuals to be followed from job security into anticipation, provides more robust evidence than has previously been available that anticipation of job loss affects health even before employment status has changed.

Adaptation, Psychological↗
PubMedDate not supplied

Analysis of longitudinal data in an Alzheimer's disease clinical trial.

Evidence of delayed progression is the primary mechanism for demonstrating therapeutic efficacy in clinical trials in Alzheimer's disease. In the major trials of therapeutic treatment of AD, to date, measures based on clinical judgement and cognitive performance, instead of mortality, have been used as the primary response measures. There is good reason for this since the course of the disease is quite long, and AD trials designed around mortality would require either very large sample sizes or very long follow-up in order to have adequate power. However, the evaluation of progression in AD using clinical markers is subject to a number of challenges often found in longitudinal databases, for example, missing data, floor and ceiling effects and non-linearity. Unfortunately, few of these issues are being addressed in the typical analysis of progression data. This paper explores these analytic issues in the context of the recently completed Alzheimer's Disease Cooperative Study trial of vitamin E and Selegeline in moderate AD patients.

Aged↗
PubMed2004

Collecting longitudinal data through childhood, adolescence, and young adulthood: methodological challenges.

This article presents the proceedings of a workshop at the 2003 Research Society on Alcoholism meeting in Fort Lauderdale, Florida. The organizers and chairs were Vivian Faden and Nancy Day. The presentations were (1) Lessons Learned From the Lives Across Time Longitudinal Study, by Michael Windle and Rebecca Windle; (2) Methodological Issues in Longitudinal Surveys With Children and Adolescents, by Joel Grube; (3) The Pittsburgh ADHD Longitudinal Study: Methodological and Conceptual Challenges, by Brooke Molina, William Pelham, Elizabeth Gnagy, and Tracey Wilson; and (4) Lessons learned in Conducting Longitudinal Research on Alcohol Involvement: If Only I Had Known Before Hand! by Kristina Jackson and Kenneth Sher.

Adolescent↗
PubMed2001

Use of longitudinal data analysis in the determination of a global score for total hip replacement indication in osteoarthritis patients.

BACKGROUND: A composite index would help in medical practice for considering total hip replacement (THR) in hip osteoarthritis. In association with other parameters, such as age or general status, this index could serve to objectively determine the time to THR surgery, and might be used as an outcome measure in therapeutic trials. The objective of this study was to propose such an index, taking into account symptomatic, structural and therapeutic variables, and impact on lifestyle. METHODS: A curve-fitting procedure and a marginal model were used to highlight the average change in the variables over time. A discrete Cox model, with time-dependent covariates. was then performed to study the association between the variables and hip surgery, leading to the construction of a composite index. RESULTS: The smoothing and fitting analysis suggested takes into account not only the variables, but also the changes in their values during the preceding year, when studying the association between variables and hip surgery. Five covariates were retained in the Cox model, among them only one concerns the changes during the preceding year, showing that the values of the covariates are more important that their changes during the preceding year for THR indication. The coefficients of the model were transformed into new coefficients, so that their sum provided a severity score with individual values 0-100. DISCUSSION: Based on the values of reliable variables recorded during the four last visits, this score can be regarded as a valid index able to objectively help with THR indications. This score might also be used as an outcome measure in therapeutic trials.

Aged↗