Growth of science and database coverage

In recent years the Web of Science (WoS) displays a tremendous growth of articles, but is unclear whether this observation is linked to a growth of science or simply an additional coverage of already existing journals by the database producer. An analysis of the category of journals in the period of 2000 to 2008 shows that the number of the basic journals from 2000 covered by WoS steadily decreases, whereas the number of new, recently established journals increases. In addition a rising number of older journals is covered in addition. These developments imply a crescive number of articles, but a relevant effect is the enlargement of the traditional, basic journals in terms of annual articles. All in all it becomes obvious that the data set is rather instable due to a high fluctuation caused by the annual selection criteria, the impact factor. In any case, it is possible to distinguish growth effects induced by the growth of science and those referring to an enlarged coverage linked to a lowering of the entry thresholds. It is important to look at the specific growth structures of a field for differentiating between "real" and "artificial" growth.