Im Zentrum der Grounded Theory als Methode zur Auswertung qualitativen Materials steht der Vergleich. Durch den ständigen Vergleich z.B. verschiedener Interviews und deren Bezug auf bereits gefundene Ergebnisse soll es möglich sein, schrittweise zu allgemeinen Aussagen fortzuschreiten.
Die Frage, wie dieser Vergleich von Statten gehen soll und wie die Gruppen ausgewählt werden, die einem Vergleich unterzogen werden, ist eine Frage, mit der sich Glaser und Strauss eingehend in ihrem Buch zur Grounded Theory, das 1967 erstmals veröffentlicht wurde, befassen. Wir zitieren daraus einige der wichtigen Stellen:
“The basic criterion governing the selection of comparison groups for discovering theory is their theoretical relevance for furthering the development of emerging categories. The researcher chooses any groups that will help generate, to the fullest extent, as many properties of the categories as possible, and that will help relate categories to each other and to their properties. Thus, …, group comparisons are conceptual; they are made by comparing divers or similar evidence indicating the same conceptual categories and properties, not by comparing the evidence for its own sake. Comparative analysis takes full advantage of the ‘interchangeability’ of indicators, and develops, as it proceeds, a broad range of acceptable indicators for categories and properties.
Since groups can be chosen for a single comparison only. there can be no definite, prescribed, preplanned set of groups that are compared for all or even most categories (as there are in comparison studies made for accurate descriptions and verification). In research carried out for discovering theory, the sociologist cannot cite the number and types of groups from which he collected data until the research is completed. In an extreme case, he may then find that development of each major category may have been based on comparisons of different sets of groups. For example, one could write a substantive theory about scientist’s authority in organizations, and compare very different kinds of organizations to develop properties associated with the diverse categories that might emerge: authority over clients, administration, research facilities, or relations with outside organizations and communities; the degree or type of affiliation in the organizations; and so forth. Or the sociologist may wish to write a formal theory about professional authority in organizations; then the sets of comparison groups for each category are likely to be much more diverse than those used in developing a substantive theory about scientists, since now the field of possible comparison is even larger.
(Glaser & Strauss, The Discovery of grounded Theory: strategies for qualitative Research, pp.49-50)