Guest Editors' Introduction: Special issue of selected papers from ECML PKDD 2011

The last edition of the European Conference on Machine Learning and Principles and Practice of Knowledge Discovery in Databases (ECML PKDD) was held in Athens, Greece, during September 5 9, 2011. The paper 'Comparing Apples and Oranges: Measuring Differences between Exploratory Data Mining Results' by Tatti and Vreeken, provides a means to highlight the differences between the results of heterogeneous exploratory data mining methods. The paper 'Diverse Subgroup Set Discovery' by van Leeuwen and Knobbe, advances the state of the art in subgroup discovery, that is the discovery of subsets of the data, where the target attributes show an interesting difference in distribution, compared to that of the entire dataset. The paper 'Finding Density-Based Subspace Clusters in Graphs with Feature Vectors' by Günnemann and co-researchers focuses on clustering networked data, where both relationships between objects as well as features of objects are considered to extract meaningful clusters.