Helkimo's index: a scale or just a set of symptoms?

This article discusses the validity of indices, exemplified by Helkimo index, by means of a critical analysis of the assumptions that should be made for constructing an index. General principles of scale construction are applied to test the adequacy of the scale-values used for five common clinical symptoms in the Helkimo Di index. These general principles postulate that in addition to identifying patients correctly, an effective scale should also discriminate normal subjects. It is concluded that there is insufficient evidence to support the general applicability of this index. Further research concerning the properties of the index is needed. It is suggested that, since Helkimo's Di index contains many valuable elements, the discussion should be directed to improving the existing scale rather than trying to construct a new one.