How the QoS performance of a communication network is affected by the disturbances, such as channel impairments link failures, node mobility, etc.? In other words, how robust is the QoS performance? Before answering these questions, we need a clear definition of QoS performance robustness and a method to quantify it. This paper contributes in the two aspects. It first proposes a new approach to define performance robustness before outlining a method to quantify and calculate robustness as a metric. Later, a case study is presented where the proposed robustness metric is used to compare two existing routing algorithms for a wireless multi-hop network