An indicator can be generally defined as an observable fact (an observable material object, property, process) that is considered as a manifestation of the existence of another fact that is not directly observable. Indicators are an important component of our life and of science, and without them we could neither work nor make decisions effectively. There must always exist a relationship between the indicator andthe unobservable fact. If such a relationship did not exist, the indicator could not be a means of obtaining knowledge of the indirectly observable facts.
The understanding of the relationships between indicators and unobservable facts is made possible by what are known as in-dicator hypotheses. These hypotheses must be rationally and empirically grounded. An indicator hypothesis operationalises our theory about the non-observable facts, and thus allows us to link our empirical experi-ence with the concepts and hypotheses that relate to facts not directly accessible.
Indicators may be divided into groups.
One distinction, also typical for educational theory, is division into qualitative and quantitative indicators. Another possible division of indicators is to distinguish between empirical and theoretical indicators. Empirical indicators are based on out empirical experience, while theoretical indicators are supported by theories, which means that they are not only grounded by a certain theoretical model, but that the latter is also independently tested.
One very important aspect is that of the possible ambiguity of indicators. If indica-tors are ambiguous (which is often the case in the social disciplines) we can reduce this ambiguity by introducing a battery of mutu-ally consistent indicator hypotheses - system of indicators, or by reducing the ambiguity of the indicator hypotheses.