This report investigates the problem of accounting co-authorship in order to quantify the impact and relevance of scientific research outputs. Here, the investigation is focused on the authors belonging to a subset of full professors of the Italian Settore Scientifico Disciplinare (SSD) FIS01 - Experimental Physics. In this SSD two populations are roughly present, characterized by the number of co-authors of each paper. Data show that the total number of citations for each individual, and consequently the individual’s h-index and g-index, strongly depends on the average number of co-authors. Here I show that, in order to remove the dependence of the various indices on the two populations, the best way to define a fractional counting of authorship is to divide the number of citations received by each paper by the square root of the number of co-authors. This normalization helps to evaluate the individual contribution in papers of experimental physics and to better understand how authors participate in the process of writing and publishing papers.