Apdex score
Apdex (Application Performance Index) is a single 0-1 score that summarises user-perceived performance: requests faster than a target T are 'satisfied' (weight 1), requests between T and 4T are 'tolerating' (weight 0.5), requests slower than 4T are 'frustrated' (weight 0). The score is (satisfied + tolerating/2) ÷ total requests.
Apdex emerged as a compromise between operational simplicity (one number) and the complexity of percentile-based SLIs. The selling point is communicability: a 0.85 Apdex is intuitively worse than 0.92 in a way that p95 numbers aren't. The trade-off is information loss: Apdex collapses the distribution into three buckets, hiding whether degradation is widespread (many tolerating) or tail-concentrated (a few frustrated). Most modern observability practices have moved away from Apdex toward direct percentile SLIs, but Apdex remains common in legacy APM dashboards and as a board-level rollup metric for less technical audiences.
Related terms
- Latency percentile
A latency percentile (p50, p95, p99, p999) is the response time below which that share of requests completed.
- SLI
A Service Level Indicator is a numerical measurement of one specific dimension of a service's behaviour — request latency, error rate, throughput, availability — expressed over a defined window.
- Real user monitoring (RUM)
Real user monitoring (RUM) instruments actual user sessions in the browser or mobile client to capture performance and error metrics as users experience them — Core Web Vitals (LCP, INP, CLS), JavaScript errors, network requests, custom timing marks.