The Source-Normalized Impact per Paper (SNIP) is a valid and sophisticated indicator of journal citation impact

Many authors have underlined that it is improper to make comparisons between citation counts generated in different research fields, because citation practices can vary significantly from one field to another. For instance, articles in biochemistry often contain over 50 cited references, while a typical mathematical paper has perhaps only 10. This difference explains why biochemical papers are cited so much more often than mathematical ones. Eugene Garfield’s view on this is clear. “Evaluation studies using citation data must be very sensitive to all divisions, both subtle and gross, between areas of research; and when they are found, the study must properly compensate for disparities in citation potential (Garfield, 1979, p 249)". Advanced citation indicators should account for differences among subject fields in the frequency at which articles cite other documents, and the rapidity of maturing and decline of citation impact.