Inter-field nonlinear transformation of journal impact indicators: The case of the h-index

[EN] Impact indices used for joint evaluation of research items coming from different scientific fields must be comparable. Often a linear transformation -a normalization or another basic operation-is considered to be enough for providing the correct translation to a unified setting in which all the fields are adequately treated. In this paper it is shown that this is not always true. The attention is centered in the case of the h-index. It is proved that it that cannot be translated by means of direct normalization preserving its genuine meaning. According to the universality of citation distribution, it is shown that a slight variant of the h-index is necessary for this notion to produce comparable values when applied to different scientific fields. A complete example concerning a group of top scientists is shown.