Kumulative Effekte zwischen Schulpraktika? Kompetenzerweiterung Lehramtsstudierender von allgemeindidaktischen zu fachdidaktischen Praxisphasen

Diese Arbeit liefert drei zentrale Ergebnisse: Zum einen wurde das Testinstrument UPDA_Math entwickelt, das die mathematikdidaktische Kompetenz (MDK) von Lehramtsstudierenden in ihrer fachdidaktischen Praxisphase erheben kann, und zwar im Paper-and-Pencil-Format mit Multiple-Choice-Items. Das dafür entwickelte Konstrukt der MDK wird durch die allgemeindidaktische Perspektive der Planung, Durchführung und Analyse von Unterricht geprägt. Das Instrument erwies sich als Rasch-skalierbar mit einer Reliabilität von 0.80. Zu den durchgeführten Modellgeltungsprüfungen zählten u.a. Tests zur Eindimensionalität, zum Differential Item Functioning zwischen leistungsstarken und leistungsschwächeren Personen, Tests auf lokale stochastische Unabhängigkeit und zur Testfairness. Als zweites wurden mit dem entwickelten Test UPDA_Math kumulative Effekte zwischen Allgemeinem Schulpraktikum (ASP) und fachdidaktischer Praxisphase (FP) im Lehramtsstudium untersucht. Es stellte sich die Frage, in welchem Ausmaß ein Erwerb hoher allgemeindidaktischer Kompetenz (ADK) im ASP einen positiven Einfluss auf den Erwerb mathematikdidaktischer Kompetenz (MDK) in der FP fördert. Als Drittvariablen wurden die Gesamtbachelornote, die Bachelornote im Fach Mathematik, die Zeit zwischen den beiden Praxisphasen, Kompetenzzuwächse im ASP und das Zweitfach kontrolliert. Entgegen der Vorannahmen, die sich aus theoretischen Überlegungen und einer qualitativen Vorgängerstudie (Roßa, 2013) ableiteten, konnte kein Zusammenhang zwischen ADK und MDK gefunden werden. Zum Teil lässt sich das mit methodischen Effekten erklären, da die beiden Ansätze der Datenerhebung zu den beiden Messzeitpunkten variierten (Selbsteinschätzungs- vs. Leistungsinstrument). Doch der nicht nachweisbare Zusammenhang zeigt auch deutlich Handlungsbedarf innerhalb der Lehrerbildung auf. Drittes Ergebnis dieser Arbeit sind Beiträge grundlegender Natur zu psychometrischen und testtheoretischen Fragen: Widersprüche des in der Literatur vorfindlichen Dimensionsbegriffs werden diskutiert und eine verbesserte Begriffsfassung vorgeschlagen. Zu den Grundlagen der Rasch-Modellierung werden Widersprüche zu den Aussagen über das Skalenniveau von Rasch-skalierten Tests untersucht. Es wird analysiert, ob und inwiefern die Testsummen im Rasch-Modell als suffiziente Statistiken bezeichnet werden dürfen. Und es werden messtheoretische Probleme des Konzepts der spezifischen Objektivität aufgezeigt. Die aufgeworfenen Fragen und einige Lösungsvorschläge seien der Forschungsgemeinschaft zur Diskussion vorgelegt.This work delivers three key results: First, the test instrument UPDA_Math was developed, which can assess the mathematics-didactic competence (MDK) of student teachers in their subject didactic practical phase. This instrument is a paper-and-pencil test in multiple-choice format. The MDK construct is characterized by the general-didactic perspective of planning, conducting and analysis of lessons. The instrument turned out to be Rasch-scalable, with a reliability of 0.80. The model validity tests carried out included tests on one-dimensionality, on the differential item functioning between high-performing persons and weaker performers, tests for local stochastic independence and test fairness. Second, with the developed Test UPDA_Math, the question of cumulative effects between general school internship (ASP) and subject didactic practical phase (FP) in the academic teacher qualification was examined. The question arose, to what extent an acquisition of high general didactic competence (ADK) in the ASP promotes a positive influence on the acquisition of mathematical didactic competence (MDK) in the FP. As third variables, the overall bachelor’s grade, the bachelor’s grade in mathematics, the time between the two practical phases, the increase in competence in the ASP and the second subject were checked. Contrary to the assumptions that were derived from theoretical considerations and a previous qualitative study (Roßa, 2013), no correlation could be found between ADK and MDK. This can in part be explained by methodological effects, since the two approaches to data collection varied at the two measurement times (self-assessment vs. performance instrument). But the undetectable connection also clearly shows the need for action within teacher training. As third result of this work there are contributions of a fundamental nature to psychometric and test-theoretical questions: contradictions in the concept of dimension found in the literature are discussed and an improved definition is proposed. For the basics of Rasch modeling, contradictions to the statements about the scale level of Rasch-scaled tests are examined. It is analyzed whether and to what extent test sums in the Rasch model can be considered as sufficient statistics. Furthermore, measurement-theoretical problems of the concept of specific objectivity are pointed out. The questions raised and some proposed solutions are presented to the research community for discussion

Multi-Stage Phase-Segregation of Mixed Halide Perovskites under Illumination: A Quantitative Comparison of Experimental Observations and Thermodynamic Models

Photo- and charge-carrier induced ion migration is a major challenge when utilizing metal halide perovskite semiconductors for optoelectronic applications. For mixed iodide/bromide perovskites, the compositional instability due to light- or electrical bias induced phase- segregation restricts the exploitation of the entire bandgap range. Previous experimental and theoretical work suggests that excited states or charge-carriers trigger the process but the exact mechanism is still under debate. To identify the mechanism and cause of light-induced phase-segregation phenomena we investigate the full compositional range of methylammonium lead bromide/iodide samples, MAPb(Br$_x$I$_{1-x}$)$_3$ with $x = 0\ldots 1$, by simultaneous in-situ X-ray diffraction and photoluminescence spectroscopy during illumination. The quantitative comparison of composition-dependent in-situ XRD and PL shows that at excitation densities of 1 sun, only the initial stage of photo-segregation can be rationalized with the previously established thermodynamic models. However, we observe a progression of the phase-segregation that can only be rationalized by considering long-lived accumulative photo-induced material alterations. We suggest that (additional) photo-induced defects, possibly halide vacancies and interstitials, need to be considered to fully rationalize light-induced phase-segregation and anticipate our findings to provide crucial insight for the development of more sophisticated models

Multi-Stage Phase-Segregation of Mixed Halide Perovskites under Illumination : A Quantitative Comparison of Experimental Observations and Thermodynamic Models

Photo- and charge-carrier-induced ion migration is a major challenge when utilizing metal halide perovskite semiconductors for optoelectronic applications. For mixed iodide/bromide perovskites, the compositional instability due to light- or electrical bias induced phase-segregation restricts the exploitation of the entire bandgap range. Previous experimental and theoretical work suggests that excited states or charge carriers trigger the process, but the exact mechanism is still under debate. To identify the mechanism and cause of light-induced phase-segregation phenomena, the full compositional range of methylammonium lead bromide/iodide samples are investigated, MAPb(BrxI1-x)3 with x = 0…1, by simultaneous in situ X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy during illumination. The quantitative comparison of composition-dependent in situ XRD and PL shows that at excitation densities of 1 sun, only the initial stage of photo-segregation is rationalized with the previously established thermodynamic models. However, a progression of the phase segregation is observed that is rationalized by considering long-lived accumulative photo-induced material alterations. It is suggested that (additional) photo-induced defects, possibly halide vacancies and interstitials, need to be considered to fully rationalize light-induced phase segregation and anticipate the findings to provide crucial insight for the development of more sophisticated models