This study is an effort to evaluate the numerical indices for the assessment of frequency response analysis (FRA) to provide a deeper understanding of their characteristics. First, the study introduces the indices in the literature and categorises them into three groups. Then, the results of 11 case studies from an experimental setup, two transformer models, and a power transformer real case are employed to examine the indices from the monotonicity aspect. Moreover, a short review on the interpretation experiences with indices is reported for further understanding. This contribution continues with a novel uncertainty analysis on the measurement setup and, finally, evaluates the indices based on their vulnerability to these uncertainties. Although the results show the advantage of some indices over the others in the FRA interpretation, further studies, especially with large transformers in the field, are needed to reach a general conclusion.