Characterizing Classes of Potential Outliers through Traffic Data Set Data Signature 2D nMDS Projection

This paper presents a formal method for characterizing the potential outliers from the data signature projection of traffic data set using Non-Metric Multidimensional Scaling (nMDS) visualization. Previous work had only relied on visual inspection and the subjective nature of this technique may derive false and invalid potential outliers. The identification of correct potential outliers had already been an open problem proposed in literature. This is due to the fact that they pinpoint areas and time frames where traffic incidents/accidents occur along the North Luzon Expressway (NLEX) in Luzon. In this paper, potential outliers are classified into (1) absolute potential outliers; (2) valid potential outliers; and (3) ambiguous potential outliers through the use of confidence bands and confidence ellipse. A method is also described to validate cluster membership of identified ambiguous potential outliers. Using the 2006 NLEX Balintawak Northbound (BLK-NB) data set, we were able to identify two absolute potential outliers, nine valid potential outliers, and five ambiguous potential outliers. In a literature where Vector Fusion was used, 10 potential outliers were identified. Given the results for the nMDS visualization using the confidence bands and confidence ellipses, all of these 10 potential outliers were also found and 8 new potential outliers were also found.