Apdex score
Apdex (Application Performance Index) is an open standard that condenses response-time measurements into a single 0–1 satisfaction score. Each sample is classified as satisfied (at or under a target T), tolerating (between T and 4T), or frustrated (over 4T). The score is satisfied plus half of tolerating, divided by total samples. It makes performance legible as one user-centric number, but the chosen T defines it.
What this means
Apdex defines a target response time T. Each measured request is satisfied if it completes within T, tolerating if it completes between T and 4T, and frustrated if it takes longer than 4T. The Apdex score is (satisfied count + tolerating count ÷ 2) ÷ total samples, producing a value from 0 to 1 where higher is better. It is an open specification, so the formula is fixed even though T is chosen per application.
Why T defines the score
The target T encodes what 'acceptable' means for a given application, so two services with identical latency can post different Apdex scores if they pick different T values. This is by design — T should reflect the experience the application aims for — but it means an Apdex number is meaningless without its T. Fix T, disclose it, and keep it constant to make the score a reliable trend.
Apdex complements RUM percentiles: percentiles show the distribution, Apdex compresses it against a stated expectation.
- Satisfied ≤ T; tolerating ≤ 4T; frustrated > 4T
- Score = (satisfied + tolerating/2) ÷ total
- Range 0–1; the target T must be disclosed
How it appears in analytics and logs
A falling Apdex means more requests are landing in the tolerating or frustrated bands relative to the target T. Because the whole score pivots on T, the threshold must be fixed and disclosed before the number is comparable across time or services.
Diagnostic use case
Summarize many response-time samples into one comparable satisfaction figure, so performance can be tracked and communicated without reading full latency distributions.
What WebmasterID can help detect
WebmasterID's observability surface measures response and interaction timings first-party, so an Apdex-style satisfaction view can be derived without third-party monitoring agents.
Common mistakes
- Quoting an Apdex score without stating T.
- Changing T and reading the score shift as a performance change.
- Treating Apdex as a replacement for the full latency distribution.
Privacy and accuracy notes
Apdex is computed from response-time samples, not user data. The timings carry no personal information when collected without identifying parameters.
Related pages
- Real user monitoring (RUM) metrics
Real user monitoring (RUM) measures web performance from actual visitors' browsers in the field, as opposed to synthetic lab testing in a controlled environment. Its headline metrics are the Core Web Vitals — Largest Contentful Paint, Interaction to Next Paint, and Cumulative Layout Shift — collected via the browser's performance APIs. Field data reflects real devices and networks, so it varies far more than lab numbers.
- Error rate
Error rate is the proportion of requests, page loads, or interactions that fail over a period. It comes in several flavors — server-side HTTP error rate (5xx/4xx share), client-side JavaScript error rate, and failed-interaction rate — each with its own numerator and denominator. As a RUM and reliability metric it signals broken experiences, but only when the failure definition and base are stated clearly.
- Average session duration and its blind spots
Average session duration is the mean length of sessions. Its core blind spot: duration is measured from the timestamps of events, so the time spent on the final page of a session — the exit page — typically counts as zero because no later event marks its end. This systematically undercounts real reading time, and GA4 replaced it with average engagement time, which is measured differently.
- Website observability
Derive satisfaction views from first-party timings.
Sources and verification notes
- Apdex Alliance — Apdex specification overviewDefines the satisfied/tolerating/frustrated bands and the score formula.
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.