Apdex

Apdex, short for Application Performance Index, is a metric used to quantify user satisfaction with the performance of software applications. It expresses performance as a score between 0 and 1, allowing teams to track and compare perceived responsiveness over time.

At the core is a target response time T, chosen to represent a satisfactory experience for most users. Each request is classified by its actual response time t into three categories: Satisfied if t <= T, Tolerating if T < t <= 4T, Frustrated if t > 4T.

The Apdex score is calculated as Apdex = (S + 0.5*Tolerating) / N, where S is the number of satisfied requests, Tolerating is the number of tolerating requests, and N is the total number of requests. A score of 1 indicates every request was satisfied, while 0 indicates none met the target.

Use and interpretation: Apdex provides a simple, intuitive summary of user-perceived performance and is often used in dashboards, incident investigations, and SLA/SLO definitions. It can be aggregated across services, endpoints, or time periods to reveal trends.

Limitations: The value depends on the chosen threshold T and on workload distribution. It compresses performance into a single number and may obscure variability or tail latency. Therefore it is typically used in conjunction with other metrics such as percentile response times.