Stable or unstable information needsThe statement that information needs change over time occurs quite frequently in information science literature. Neither "change" nor "over time" is, however, clearly defined and from the context of the use of the phrase it is clear that different authors interpret both concepts differently. Most of the articles are purely descriptive and don"t try to offer a theoretical discussion. In this paper we give an overview of examples as they occur in recent publications and discuss all variables that impact on the interpretation of these terms.The often-cited article by Belkin and Croft (1992) deals primarily with the relation between information filtering and information retrieval. They conclude that "information retrieval and information filtering are indeed two sides of the same coin. They work together to help people get the information needed to perform their tasks" (Belkin and Croft 1992: 37). One of the characteristics of information filtering, according to them, is that[i]nformation filtering is concerned with much larger data sets, and, generally, with information needs which are relatively stable over relatively long periods of time (Belkin and Croft 1992: 37).For this discussion the second part of this quotation is of primary importance: if there are information needs that are relatively stable over relatively long periods of time, the logical deduction is that there are also information needs that are relatively unstable over long or short periods of time. Instability implies change, i.e. it is safe to deduce that their description implies that information needs may change over time (regardless of the length of the time period). This is to a certain extent substantiated by the following quotation from Belkin and Croft (1992): relatively stable, long-term, or periodic goals or desires (e.g., accomplishing a work task, or being entertained) [...] then lead to regular information interests (e.g., keeping up-to-date on a topic) that may change slowly over time as conditions, goals, and knowledge change (Belkin and Croft 1992: 31).Even though they here refer to interests and not needs, the statement still supports the preceding deduction, as information interests imply or lead to information needs. They also imply that the changing information needs could be triggered by conditions external or internal to the user of the information: conditions can refer to the context or situation of the information need; goals are internal in the user of the information and knowledge could be either the knowledge of the user or refer to the "state of knowledge" (e.g. changes in factual knowledge as the world changes). Belkin and Croft (1992), however, do not discuss the issue of changing information needs any further. As this issue is not the focus of their article, they do not discuss either the nature of such changes in information needs or the concept "over time" in any detail. Does change, for example, refer to a gradual change in an existing information need, or does change imply a new...