IRTrees: Tree-Based Item Response Models of the GLMM Family

A category of item response models is presented with two defining features: they all (i) have a tree representation, and (ii) are members of the family of generalized linear mixed models (GLMM). Because the models are based on trees, they are denoted as IRTree models. The GLMM nature of the models implies that they can all be estimated with the glmer function of the lme4 package in R. The aim of the article is to present four subcategories of models, the first two of which are based on a tree representation for response categories: 1. linear response tree models (e.g., missing response models), 2. nested response tree models (e.g., models for parallel observations regarding item responses such as agreement and certainty), while the last two are based on a tree representation for latent variables: 3. linear latent-variable tree models (e.g., models for change processes), and 4. nested latent-variable tree models (e.g., bi-factor models). The use of the glmer function is illustrated for all four subcategories. Simulated example data sets and two service functions useful in preparing the data for IRTree modeling with glmer are provided in the form of an R package, irtrees. For all four subcategories also a real data application is discussed

The Estimation of Item Response Models with the lmer Function from the lme4 Package in R

In this paper we elaborate on the potential of the lmer function from the lme4 package in R for item response (IRT) modeling. In line with the package, an IRT framework is described based on generalized linear mixed modeling. The aspects of the framework refer to (a) the kind of covariates -- their mode (person, item, person-by-item), and their being external vs. internal to responses, and (b) the kind of effects the covariates have -- fixed vs. random, and if random, the mode across which the effects are random (persons, items). Based on this framework, three broad categories of models are described: Item covariate models, person covariate models, and person-by-item covariate models, and within each category three types of more specific models are discussed. The models in question are explained and the associated lmer code is given. Examples of models are the linear logistic test model with an error term, differential item functioning models, and local item dependency models. Because the lme4 package is for univariate generalized linear mixed models, neither the two-parameter, and three-parameter models, nor the item response models for polytomous response data, can be estimated with the lmer function.

Spider phobics more easily see a spider in morphed schematic pictures

Background: Individuals with social phobia are more likely to misinterpret ambiguous social situations as more threatening, i.e. they show an interpretive bias. This study investigated whether such a bias also exists in specific phobia. Methods: Individuals with spider phobiaor social phobia, spider aficionados and non-phobic controls saw morphed stimuli that gradually transformed from a schematic picture of a flower into a schematic picture of a spider by shifting the outlines of the petals until they turned into spider legs. Participants' task was to decide whether each stimulus was more similar to a spider, a flower or to neither object while EEG was recorded. Results: An interpretive bias was found in spider phobia on a behavioral level: with the first opening of the petals of the flower anchor, spider phobics rated the stimuli as more unpleasant and arousing than the control groups and showed an elevated latent trait to classify a stimulus as a spider and a response-time advantage for spider-like stimuli. No cortical correlates on the level of ERPs of this interpretive bias could be identified. However, consistent with previous studies, social and spider phobic persons exhibited generally enhanced visual P1 amplitudes indicative of hypervigilance in phobia. Conclusion: Results suggest an interpretive bias and generalization of phobia-specific responses in specific phobia. Similar effects have been observed in other anxiety disorders, such as social phobia and posttraumatic stress disorder. © 2007 Kolassa et al; licensee BioMed Central Ltd

1. Session (02.04.2007)

Vorlesungsinhalt: Einführung in R; Erste Schritte; Objekte in

4. Session (01.04.2007)

Vorlesungsinhalt: Einfache Grafik; Low-level commands; Trellis Grafiken; Formeln; Modelle; generalisierte lineare Modell

The Presidential Institution in Bulgaria, Albania and Romani: An Attempt at Comparison"

Bulgaria, Albania and Romania are all parliamentary republics with a president as head of state. Although the Albanian president is elected by Parliament, he is arguably the strongest of the three, both in terms of the powers allowed by the provisional constitution and of Mr. Berisha's political practice. The constitutional reform underway in the country will however change the status quo. In Bulgaria and Romania the presidents are elected directly by popular vote, but their powers are relatively small as compared to the democratic legitimisation implied by direct elections. Actual presidential powers should however be assessed with caution as some of them are set by law or interpretations of constitutional texts, rather than by the constitutions themselves. There is also variation in the degree to which the presidents in office have exploited their constitutional powers or taken their role as non-aligned political brokers seriously. Mr. Berisha, in particular, was in control of party politics throughout his presidency and was one of the most polarising influences on public opinion. The excessive political polarisation in all three countries has however its own logic and power. Thus Mr. Zhelev invariably supported the emergence of a political centre in Bulgaria, but this did not succeed and the policy was as damaging to his political career as the fight with would-be centrists was to Berisha's. Political practice in all three countries seems to need a presidential figure. This adds flexibility to a situation governed by hostile and mutually suspicious parties, stuck parliaments and weak or inexperienced governments. The presidents also command considerable influence on public opinion. Public opinion in Bulgaria, for example, largely supports the idea of greater power for the president, in contrast with the opinions of constitutionalists and other law-makers in the country. Under the legacy of the past, the people have a love-hate relationship with such paternalist figures. Presidents personalise politics in the public mind, but they can also become scapegoats for political failures

5. Session (03.04.2007)

Vorlesungsinhalt: generalisierte lineare Modell