In a very general view the concept situation describes the circumstances, the current conditions, and state someone is in. In this thesis the term situation will be used in this general form, based on the following definition from an observer’s viewpoint.
Definition: Situation
A situation is the state of the real world at a certain moment or during an interval in time at a certain location.
This general definition of situation has implications when it comes to the task of describing a situation. To fully describe the state of the real world at a given moment in time is almost impossible. Any description of a situation is therefore incomplete. A description is hence always an abstraction of the real world; the real complexity of the situation is reduced to the characteristic of the situation. The step to select the characteristic properties of a situation is always subjective. The decision which aspects are characteristic for a certain situation is left to the person who makes the description. Unstructured descriptions for the same situation made by different people will most often result in diverse descriptions. Different levels of abstraction (e.g. forced by different maximal length) of a description will inevitably lead to different reports on the same situation. A further and important issue is that a description is also dependent on the goal of the person who produces the description. In general it can be observed from daily experience that humans characterise and describe situations differently based on their role, task, goal, expectations, emotions, and knowledge. Nevertheless descriptions are the most used means in everyday life to communicate knowledge about situations, well knowing that these descriptions are neither objective nor complete.
More structured ways of describing situations are questionnaires and check-lists. Using these mechanisms the characteristic features of a situation that are of interest are predetermined by the person producing the questionnaire or check-list. To some extent the level of abstraction that is expected from the person who fills the form is determined by the space (e.g. number of lines, words expected, text box size) provided.
Matching similar situations is a task that is essential in life. By recalling similar situations creatures have means to transfer accumulated knowledge and experience from the past and gain therefore advantage. E.g. a car driver recognises a situation where there is ice on the street and proceeds slower because the driver knows from previous experience or knowledge that driving fast in this situation may be dangerous and could result in an accident. This even works if the driver has never travelled this particular road before, because for this type of situation it is not important that this is a specific road. This example explains a further challenge: the description of a situation has to be specific enough to match only relevant situations but as general as possible to match all similar and related situations where the knowledge is useful.
Definition: Context
A Context is identified by a name and includes a description of a type of situation by its characteristic features.
The description can constitute of a number of conditions that can be evaluated to true or false, possibly with an assigned certainty. Having such a context it can be evaluated whether or not a specific situation belongs to that context, which is described. Context is a mechanism to describe situations by their defining features and group them into one unit. In certain cases the description can consist of a single complex condition that is not necessarily human readable, e.g. an artificial neural network.
Definition: Situation S belongs to a Context C
A situation S belongs to a context C when all conditions in the description of C evaluate to true in a given situation S.
The following properties are desired for the description of a context:
a) All situations that are of the described type should be matched by the description
b) A situation that does not belong to the described type should not be matched c) Given a) and b) the description should be minimal
Creating a description of a context includes similar problems to creating a query for information retrieval. To assess the quality of a description measures such a precision
and recall, well known from information retrieval can be used [Baeza-Yates,99]. Based on these definitions context can be regarded as a pattern, which can be used to match situations of the same type.
It is obvious that a situation in the real world can belong to many contexts. This is an observation that is coherent with experience in everyday life. Most often a situation does not solely belong to one type of situation. Therefore situations in the real world will be treated as belonging non-exclusively to a context. That means a certain situation can belong to none, one, or a number of contexts.
To reduce complexity it is possible to create a restricted model which only allows exclusive contexts. The set of contexts has to be selected in a way that possible situations are exclusive of each other. This makes the development of perception systems as well as context-aware applications much easier.