## 2 The Tendency to Increase the Volume and Complexity of Data and Program Collection of the DSS
Achieving an efficient operation of each such sub-system, as well as maintaining a proper communication among the individual systems, while keeping the large data and program core healthy and consistent, is a difficult task indeed. 2. The huge numbers of external data sources of large DS systems: In a considerable number of cases, the large DSS data core must handle data from a huge number of different, heterogeneous and autonomous external sources, such as databases and/or various data collections. For example, the European Transport Information System for Policy Support (ETIS) receives data from more than 500 external sources [20], which are indeed heterogeneous and autonomous. Handling this number of different sources is a very difficult task and, in most cases, tackling this task is by no means automatic. Indeed, usually, a large initial effort by experts is required, in order to create a first version of an interoperable, internally consistent data and program core [34]. * In a large data collection, a considerable number of differences in the meaning and/or definition of common variables, coming from different sources, may appear. Thus, the task of defining the exact meaning and form of the variables that are common to many sources of the DS system may prove to be extremely difficult. For example, in the first stages of the ETIS design, it has been proved that the difficulty of defining the term/variable \"long distance trips\" [35] was almost insurmountable. In fact, for the Scandinavians, this term referred to a distance greater than 100 km, while such an option practically excluded trips in the Netherlands entirely. In addition, other countries expressed a strong desire to associate with this term the age of the passengers and/or to incorporate in it whether the relevant trip is transit or not. Today, one may think that the term \"long distance trips\" must also include as a sub-variable the information whether the vehicle (automobile, motorcycle or train) is electric or conventional. Thus, it not a surprise that a new, complete definition of the term \"long distance trips\" was given by the Transport General Directory and that Eurostat issued a directive to all EU Members to conform with this new definition and to always provide all relevant information. * The next difficulty concerns the \"level of detail\" (abbreviated as LoD) or \"granularity\". We shall try to clarify the content and importance of the term \"level of detail\" by means of the following example, concerning Google Maps. In fact, this application starts from offering a global earth projection. Then, the user may gradually increase the level of detail at an arbitrary point of the projection, according to his/her desire. At each such selection of the user, a more detailed map of the associated area appears on the screen. At certain levels, new geographical objects appear on the screen. For example, one may select a specific country and see its map. Subsequently, one may choose an area of this country and see the corresponding map in more detail. After a sequence of successive selections, where each one of them offers a greater level of detail, one may suddenly see some cities as points and some sketches of roads; hence, new objects appear. Further increase of the detail may offer more and more dense maps of the desired area. If one selects the option \"satellite\", more and more detailed images of the selected area appear on the screen. In the higher detail level, the user may see individual public buildings and constructions, houses and details of them. In the first steps of large DS system implementation, many designers considered that it is sufficient to include the higher level of detail only in the information system. However, the two aforementioned examples, but also many other DS systems that do not necessarily include geographical information, demonstrate that the approach \"include the highest level of detail only\" is completely erroneous.