International effective school research outlines the central impact principals have on the quality of their school. Schools that are referred to be successful have principals that show professional leadership. Regarding the complexity of tasks principals are dealing with, leadership skills and competencies are needed for succeeding in their job (e.g. Bonsen 2006, Huber 2008). A well established instrument to identify suitable prospective leaders in general is the assessment center (Lievens & Thornton 2007). Internationally, assessment centers (AC) are considered to be appropriate for selecting prospective principals (Chapman 2005). England or the USA for instance gained a lot of experience in selecting future principals by assessment center (Huber & Pashiardis 2008). In Germany, the school system is under federal control (ibid.) and therefore, the procedures of acquiring future principals differ widely. The use of assessment center for recruiting and developing principals is slowly increasing (Klein 2005, Buhren & Rolff 2002).

North Rhine-Westphalia is the first federal state in Germany that introduced assessment center as an obligatory policy for selecting prospective principals. The ministry of school and further education of North Rhine-Westphalia highlighted the competences innovation, management, communication, and role clarity as crucial for successful leadership and specified them for the working context of principals. Since 2009, teachers have to show the defined competences in a two-day assessment center (EFV) before they are approved to apply for a principal position.

This innovative instrument has been evaluated in 2011 (Ortenburger & Kuper) and 2012, focussing on the objectivity, reliability and validity of the assessment center. Descriptive and multivariate analyses have been conducted with data of assessors and participants as well as with the results of the assessment center. The inter-rater reliability based on postexercise dimension ratings is relatively high and varied within the range of 0,626 and 0,893 (n=863, Spearman correlation). For the verification of construct-related validity, a multitrait multimethod matrix was analyzed. Conform to assessment center research (e.g. Woehr & Arthur 2003), the correlations of the same dimensions in different exercises (convergent validity) were low (0,110 - 0,193; Spearman correlation) while the correlations of different dimensions in the same exercise were high (discriminant validity) (0,585 - 0,780; Spearman correlation). These results argue against the existence of construct-related validity.

Following the argumentation of Lance (2007 and Lance at al. 2000) such findings in discriminant validity don’t have to stand for a method or observer bias, but can be understood as cross-situational specific performance of the participants. As empirical research showed, exercise effects are related to external performance-related variables and should be interpreted as true values (ibid.). As the assessor ratings in the EFV are quite consistent and discriminant validity is high, the question arises, if the cross-situational specific performance underlying the strong exercise effect can be characterized by the assessors.

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