In a given field there are essentially two types of articles, N-author articles and single author articles. Related to this fact, I wonder how a committee when evaluating someone for a tenure promotion establishes the personal contribution of an N-authors article? Just to give an example, suppose that we have two candidates which one has in total 20 articles, all co-authored, and another candidate which has in total 15 articles, with only two co-authored articles, and the rest are single authored. Assume also that both candidates have published their work in the best journals in their working field.
Based on your experience and on my example above, how is in reality determined the PERSONAL contribution of an N-authors article? Would this contribution just be 1/N? Which of the candidates in the example which described above would be more likely to be selected for a promotion? How in reality are seen single author articles published in the best journals in a given working field?
An observation: Usually people who work with several co-authors tend to have more publications than someone who works alone. Also in many fields, in an N-authors article, the author names are put in alphabetic order.
EDIT NR.1: My question is mostly related to theoretical fields, such as mathematics, theoretical physics and computer science. In these fields usually the number of co-authors is N<10.