We have developed a formula that assigns relative values to each author of the list in any publication according to the authors' relative positions. The formula satisfies several criteria of theoretical and practical significance. We tested the formula's validity and usefulness with bibliographical references from the INSPEC database, mainly from the physical sciences. Enforced alphabetical sorting, different names of single authors and other statistical disturbances are accounted for. Our results demonstrate that our formula, or any other that satisfies several objective and quantitative criteria, can and often should be used as an additional criterion in the processes of evaluating relative scientific productivity, detecting experts in a given discipline, etc.