Biomedical subjects

A K Formann

Publications and source records attributed to A K Formann.

7 recordsLinked to original sources

PubMed2001

Neurodevelopmental schizophrenia: obstetric complications, birth weight, premorbid social withdrawal and learning disabilities.

Neurodevelopmental schizophrenia seems to be caused by impaired cerebral development and is supposed to be associated with obstetric complications (OCs), poor premorbid adjustment, schizotypal or schizoid personality traits and negative symptoms. In the present study, 36 schizophrenic and schizoaffective patients and their same-sex, healthy siblings were recruited. They were diagnosed according to DSM-III-R, using structured psychiatric interviews and a consensus of 2 psychiatrists. Information on OCs, birth weight, premorbid social and learning functioning was obtained from their mothers. The main results show significant differences in OCs, birth weight, premorbid social and learning functioning between patients and their same-sex, healthy siblings. Using multivariate analyses, both premorbid variables were again identified to discriminate well between affected and unaffected siblings. Our findings seem to confirm the concept of schizophrenia as a neurodevelopmental process.

Adolescent↗

PubMed2001

Misspecifying latent class models by mixture binomials.

Four scenarios of homogeneity/heterogeneity with respect to the performance of the subjects and the task difficulties are considered: first, the unconstrained latent class model providing for heterogeneity with respect to both; second, the mixture binomial assuming constant task difficulty within each mixing component, but different levels of performance of the subjects; third, the model of independence which is equivalent to the one-class latent class model allowing for different task difficulties but no variability of the subjects; and fourth, the binomial with success probability constant across tasks and subjects. It is shown that both over- and underdispersion may arise in latent class models of which the other three are special cases. As a consequence, the latent class model and the mixture binomial may generate nearly indistinguishable score distributions where overdispersion is present. So the score distribution is not always indicative of the lack of fit of the mixture binomial when in fact the latent class model is true. It may, therefore, be misleading to accept mixture binomials as well-fitting models without having additionally assessed the fit of latent class models. This, however, is often the case in empirical research. A long series of investigations on Piaget's water-level tasks serves as a good example.

Adolescent↗

PubMed2000

Rater agreement and the generalized Rudas-Clogg-Lindsay index of fit.

Rudas, Clogg and Lindsay proposed a new index of fit for contingency table analysis. Using the two-component mixture, where the first component with weight (1-w) represents the model to be tested and the second component with weight w is unstructured, the RCL index of lack of fit was defined to be the smallest mixing weight w(*) being compatible with the two-component mixture to be saturated. This index of fit, which is not sensitive to sample size, is applied to the problem of assessing agreement between two raters whereby three hypotheses (pure agreement, quasi-independence, independence) are considered. As quasi-independence comprises the two other hypotheses, a natural generalization of the RCL index of fit results from assuming the model itself to be composed of two submodels (pure agreement, independence) while the third component remains unstructured. Two examples demonstrate the application of this generalized RCL index of fit, with the first 3x3 contingency table having no empty cells, and the second 4x4 table having five of them. In both cases estimating the parameters and determining w(*) was possible without problems within the linear logistic framework for latent class analysis. A further analysis of the 4x4 table assumes the model to be the two-class latent class model that certainly does not belong to the family of standard models for contingency table analysis. Thus, in contrast to the recommendations given originally, the following conclusions seem to be justified: (i) the number of components representing the model may exceed one (generalized RCL index); (ii) the application of the original RCL index of fit may be extended to more complex models; and (iii) empty cells bear no problems so that this approach may be recommended in the case of both large and small sample sizes.

Dermatitis, Atopic↗

PubMed1998

Quality of life and psychosocial adjustment of young patients after treatment of bone cancer.

BACKGROUND: The aim of this study was to collect information about the psychosocial situation of young patients after multimodality therapy for bone cancer. METHODS: Selection criteria for patients were ages 15-30 years, tumor localization at the extremities, and an interval of at least 1 year since the end of treatment. Of 110 patients, 60 were willing to participate. Evaluation of psychosocial quality of life included assessment of psychosocial adjustment and age-appropriate achievements as well as identification of problems typical for this patient group. RESULTS: Approximately 80% of patients revealed, at the very most, only minor psychosocial problems. They were able to adapt well to their new living conditions, although strong efforts were necessary for them to deal with problems such as restricted mobility, catching up with school, or changing jobs or job orientation. Differences between patients and control subjects emerged in the areas of marital status, independent living, and parenthood. The most recently determined levels of education and income were similar. Neither clinical data nor physical or functional sequelae affected psychosocial adjustment, with one exception: patients diagnosed in adolescence had significantly more problems, especially in the area of social well-being, than patients diagnosed in childhood or early adulthood. CONCLUSIONS: Given the limitations of this study, the findings suggest that survivors of bone cancer are not necessarily at risk of developing long term emotional or social problems and are not precluded from leading active and independent lives.

Adaptation, Psychological↗

PubMed1996

Latent class analysis in medical research.

In the introduction we give a brief characterization of the usual measures for indicating the quality of diagnostic procedures (sensitivity, specificity and predictive value) and we refer to their relationship to parameters of the latent class model. Different variants of latent class analysis (LCA) for dichotomous data are described in the following: the basic (unconstrained) model, models with parameters fixed to given values and with equality constraints on parameters, multigroup LCA including mixed-group validation, and linear logistic LCA including its relationship to the Rasch model and to the measurement of change in latent subgroups. The problem with the identifiability of latent class models and the possibilities for statistically testing their fit are outlined. The second part refers to latent class models for polytomous data. Special attention is paid to simple variants having fixed and/or equated parameters and to log-linear extension of LCA with its possibility for including on the latent level. Several examples are presented to illustrate typical applications of the model. The paper ends with some warnings that should be taken into consideration by potential users of LCA.

Clinical Trials as Topic↗

PubMed1994

Measurement errors in caries diagnosis: some further latent class models.

Latent class models have been proposed for assessing relative errors in discrete measurements arising, for example, in caries diagnosis. Those models, however, that turned out to fit empirical data (N = 3,869 teeth, 5 dentists) to a sufficient degree, needed the inclusion of interaction terms for pairs of raters at the latent levels in order to describe the observed interrelations of the judgments. Now it is shown that instead of giving up the concept of local stochastic independence, increasing the number of classes while possibly restricting their parameters in the end has the same effect: Both the unrestricted three-class model, which can be interpreted to be the generalization of the Carlos-Senning assumptions, and a restricted four-class model, which resembles the latent distance model well known from psychometrics, give good and excellent fit, respectively, to the caries data. For the case of the four-class model, a second solution exists that exhibits exactly the same fit, giving rise to warnings against the possibility of multiple solutions to the likelihood equations.

Biometry↗

PubMed1993

Fixed-distance latent class models for the analysis of sets of two-way contingency tables.

This paper develops and describes the application of modified latent class models for analyzing sets of two-way contingency tables. The proposed fixed-distance models differ from traditional latent class models in that the assumption of local stochastic independence is superseded by allowing interactions of the manifest variables within each class, which can be represented by a single association parameter. As an example, two data sets on eye color by hair color [collected in Caithness (N1 = 5,387) and Aberdeen (N2 = 22,361)] and fixed-distance models with up to six classes (three classes per data set) are considered, finally leading to satisfactory fit and rather simple interpretation.

Child↗