Biomedical subjects

Geert Verbeke

Publications and source records attributed to Geert Verbeke.

5 recordsLinked to original sources

PubMed2003

A comparison of group sequential methods for binary longitudinal data.

Interim analyses are conducted to allow for early termination of the trial, for ethical as well as economical reasons. Here we consider interim analyses in repeated measurements studies where the measurements are binary. Two methods for analysing this kind of data are compared according to their operating characteristics. A subject-specific approach based on the logistic random-effects model is compared with the population-averaged approach based on the generalized estimating equations. The comparison is illustrated with simulations using a randomized clinical trial for toenail fungal infection.

Antifungal Agents↗

PubMed2003

The use of score tests for inference on variance components.

Whenever inference for variance components is required, the choice between one-sided and two-sided tests is crucial. This choice is usually driven by whether or not negative variance components are permitted. For two-sided tests, classical inferential procedures can be followed, based on likelihood ratios, score statistics, or Wald statistics. For one-sided tests, however, one-sided test statistics need to be developed, and their null distribution derived. While this has received considerable attention in the context of the likelihood ratio test, there appears to be much confusion about the related problem for the score test. The aim of this paper is to illustrate that classical (two-sided) score test statistics, frequently advocated in practice, cannot be used in this context, but that well-chosen one-sided counterparts could be used instead. The relation with likelihood ratio tests will be established, and all results are illustrated in an analysis of continuous longitudinal data using linear mixed models.

Analysis of Variance↗

PubMed2002

Impaired tolerance for glucose in women with recurrent vaginal candidiasis.

OBJECTIVE: The purpose of this study was to determine whether nondiabetic women with recurrent vaginal candidiasis have an impaired glucose metabolism. STUDY DESIGN: A case-control study of 62 otherwise healthy women who were attending a vaginitis clinic > or =3 times a year for symptoms of Candida vaginitis, positive microscopy, and at least one positive Candida culture and of 32 Candida-negative control subjects, all of whom were undergoing standardized oral glucose tolerance testing. RESULTS: Women with recurrent bacterial vaginal infections did not differ from women without infections, so both groups comprised the control group. Women with recurrent vaginal candidiasis had a greater mean body mass index than the control subjects (23.5 vs 21.4, P =.001). They had no more incidences of overt or preclinical diabetes mellitus than the control subjects (6/62 vs 0/32 incidents), but a greater proportion of them had at least one glucose concentration above the 95th percentile (36% vs 12%, P =.016). Glucose concentrations were higher in recurrent vaginal candidiasis cases than in control subjects at 0 (89 vs 85 mg/dL,P =.049), 30 (139 vs 126 mg/dL, P =.05), and 60 minutes (123 vs 102 mg/dL, P =.009) after the intake of 75 g of glucose. Fasting concentration of glycosylated hemoglobin was 25% higher in women with recurrent vaginal candidiasis (5 vs 4 g/dL, P =.0006), even after correction for body mass index. Compared with control subjects, ingestion of 75 g of glucose led to a 15% increase of serum glucose levels in women with recurrent vaginal candidiasis (P =.01). As expected, most of these differences were largely mediated by an increased body mass index. CONCLUSION: The tolerance to glucose in nondiabetic women with recurrent vaginal candidiasis is discretely impaired. Glucose tolerance testing is of value in women with recurrent vaginal candidiasis, but the interpretation of the obtained values should not be limited to the diagnosis of preclinical diabetes mellitus.

Adult↗

PubMed2002

Strategies to fit pattern-mixture models.

Whereas most models for incomplete longitudinal data are formulated within the selection model framework, pattern-mixture models have gained considerable interest in recent years (Little, 1993, 1994). In this paper, we outline several strategies to fit pattern-mixture models, including the so-called identifying restrictions strategy. Multiple imputation is used to apply this strategy to realistic settings, such as quality-of-life data from a longitudinal study on metastatic breast cancer patients.

Journal Article↗

PubMed2002

Group sequential methods for an ordinal logistic random-effects model under misspecification.

Interim analyses in clinical trials are planned for ethical as well as economic reasons. General results have been published in the literature that allow the use of standard group sequential methodology if one uses an efficient test statistic, e.g., when Wald-type statistics are used in random-effects models for ordinal longitudinal data. These models often assume that the random effects are normally distributed. However, this is not always the case. We will show that, when the random-effects distribution is misspecified in ordinal regression models, the joint distribution of the test statistics over the different interim analyses is still a multivariate normal distribution, but a sandwich-type correction to the covariance matrix is needed in order to obtain the correct covariance matrix. The independent increment structure is also investigated. A bias in estimation will occur due to the misspecification. However, we will also show that the treatment effect estimate will be unbiased under the null hypothesis, thus maintaining the type I error. Extensive simulations based on a toenail dermatophyte onychomycosis trial are used to illustrate our results.

Biometry↗