Purpose: The purposes of this paper are, first, to establish the psychometric properties of the ELP tool, and, second, to test, using a Rasch item response theory analysis, the hypothesized progression of challenge presented by the items included in the tool. Design/ Methodology/ Approach: Data were collected at two time points through a survey of the educational leadership practices of school principals (n = 148) and their teachers (n = 5,425). The survey comprised seven effectiveness scales relating to school-wide dimensions of leadership, and one scale relating to the effectiveness of individual principals' leadership. The authors undertook validation of the hypothesized structure of the eight ELP scales using the Rasch rating scale model. Findings: The authors established constructs that underpin leadership practices that are more and less effectively performed and determined the nature of their progression from those that are relatively routine through those that are more rigorous and challenging to enact. Furthermore, a series of analyses suggest strong goodness-of-model fit, unidimensionality, and invariance across time and educator group for the eight ELP scales. Research Limitations/Implications: This study focussed on experienced principals--future studies could usefully include school leaders who are new to their role or compare leadership patterns of higher and lower performing schools. A useful future direction would be to investigate the predictive validity of the ELP tool. Originality/Value: This study reveals the ELP is a useful tool both for diagnosing leadership effectiveness and, given that it is essentially stable over time, may prove useful for charting the effectiveness of leadership development interventions.