FAQ  〉What is APDEX?

Apdex is a client perception based performance metric. It buckets requests in 3 categories depending on their response time: Satisfied, Tolerating and Frustrated.
It then computes a single number between 0 and 1 (higher the better) using the formula shown below.
Computing the Apdex requires a single parameter T "threshold" used to draw the line (in term of response time) between the 3 buckets: all requests faster than T are Satisfied, all requests between T and 4×T are Tolerating and all other requests (above 4×T or failed) are considered Frustrated.

diagram explaining the previous definition with an example

Apdex is usually applied server-side, measuring app response time only, but it is totally possible to compute it client-side like updown.io does. In this case it also includes network latency, which is a good thing because your users too, but it requires higher thresholds to take this into account.

The Apdex threshold can be configured individually for each check:

screenshot of updown UI showing the Apdex threshold selector

And any change to this value will immediately impact the dashboard and status page charts.

We recommend setting the Apdex T just above your usual response times, for example in this case 0.5 would be a good value:

screenshot of and updown status page response time chart

Adrien Rey-Jarthon
Created on October 17, 2023