## 2 The Tendency to Increase the Volume and Complexity of Data and Program Collection of the DSS  
A considerable number of contemporary DSS applications eventually ask for very large data and program collections. The tendency to increase the size and complexity of these collections is due to a variety of factors, such as:  
1. Advances in information technology and communications (ITC) have created a market for DSSs with very large-scale data and program collections. In fact, progress in software design and implementation makes possible the almost instant handling of huge data sets. For example, the conceptual development of balanced, N-ary trees allowsfor accessing a specific entry among trillions of entries in four or five very fast steps only. In addition, contemporary hardware allows for creating and managing/handling \"storages\", that is, devices that may contain thousands of disks (hard disks or DSSs). 2. The same ITC advances have opened the way to engineers to attempt to develop DSSs that tackle very big and/or complicated practical problems. Two related examples follow: * The effort for developing the European Transport Information System for Policy Support (ETIS). Maintaining an efficient transport infrastructure and formulating a common transport policy are critical elements for the economic and social development of the European Union (EU). In 1994, the EU Commission adopted a comprehensive proposal for the \"Trans-European Networks\" (TEN-T) guidelines [29], which are plans for improving the performance and further development of the complex EU infrastructure. The first version of these guidelines covered 70,000 km of rail tracks (including 22,000 km of new and upgraded tracks for high-speed trains), 58,000 km of roads (including 15,000 km of new roads), corridors and terminals for combined transport, 267 airports of common interest, networks of inland waterways and many seaports. It is evident that these numbers have greatly increased since the adoption of TEN-T guidelines. The effort for performance improvement of the Trans-European Networks and the application of a successful transport policy requires significant information concerning the current status of the networks and the corresponding inhabited regions. More specifically, this required information includes passenger and freight transport volumes, traffic congestion, environmental impacts of transport, etc. It is also necessary to consider reliable forecasts for the time evolution of the aforementioned factors. The relevant information exists in a large number (more than 500) of dispersed, heterogeneous and autonomous databases throughout the entire Europe. In addition, more relevant information is acquired from the results of an ensemble of pertinent scientific studies. The European Transport Information System for Policy Support has been envisaged as a tool for the support of decision making on transport policies and policies related to the Trans-European Transport Networks. ETIS must accommodate policy-related information in a repository that has to be kept up to date by experts. In ETIS, policy questions are linked to the relevant data sets of the sources via a hierarchical structure of policy criteria, policy indicators and data variables [20]. * A second example is the information system for the port of Rotterdam. This system includes a huge amount of geographical information. It is noteworthy that the port of Rotterdam incorporates more than 5000 docs, an airport, a considerably large storage area for alumina and abundant apron spaces for temporary storage of goods and containers. The IS of the port of Rotterdam, called Port Community System (PCS), provides services that focus on all port sectors, such as containers, break bulk, dry bulk and liquid bulk. Anyone in the logistics chain can easily and efficiently exchange information through these services. Moreover, the PCS offers a package of properly tailored services to each of the target groups of clients and operators.