This work is concerned with defining a performance index that can be used as an objective measure in the evaluation and comparison of ad hoc networking protocols. Specifically, we introduce a performance index that aggregates performance results from multiple response metrics into a single scalar value that quantifies overall system performance, leading to more objective evaluations and comparisons. Using a statistical-based experimental design and analysis strategy, we illustrate the advantages of the proposed performance index by comparing two ad hoc networking systems over a wide range of networking scenarios. Based on the results of this analysis, we develop an empirical model that characterizes the relationship between the proposed performance index, four response metrics (packet delivery ratio, end-to-end delay, routing overhead, and jitter) and four influential factors (node mobility, offered load, network size, and routing protocol).