BACKGROUND Solvent extraction is one of the most used processes worldwide for metal extraction at an industrial and laboratory scale. As a process that requires an external component, the selection of extractants is one of the most studied topics to find the most suitable ones for a particular case. The use of combined reagents, or so‐called synergistic extractants, is a branch of extraction studies that have proven better results than the use of solvents on their own. However, a comparison between synergistic extractants cannot be made with the currently reported factors. This paper provides a guide on clarifying the implications of the conventional synergistic factor, R, for the evaluation of the best extractant combinations. Moreover, a new indicator is proposed, named relative distribution enhancement factor, H, that will quantitatively measure the efficiency of the extraction and compare extraction enhancement among different experimental research. RESULTS Four references were selected to illustrate the use of the H factor in contrast with the R factor. The usefulness of the new enhancement factor H was demonstrated by evaluating the synergistic solvent extraction of rare earth elements. It was found that the use of R factors can mislead the selection of extractants ratio, and although synergism exists this does not imply an enhanced extraction. CONCLUSION The main finding of this work is that the newly developed H factor can easily detect extraction enhancement or diminishment and is not limited to the evaluation of two extractants. © 2021 Society of Chemical Industry (SCI).