All Products
Document Center

Statistical metrics

Last Updated: Apr 12, 2018


Application Performance Index Apdex is an international application performance calculation standard. According to Apdex, user experience of an application can be divided into three levels:

  • Satisfactory (0 to T)
  • Tolerating (T to 4T)
  • Frustrated (greater than 4T)

The calculation formula is:

  1. Apdex = (Satisfactory samples + Tolerating samples/2)/Total samples


Image source:

For ARMS, First Paint Time is taken into account. The default T value is 2 seconds.

JS stability

In ARMS, JS stability refers to the JS error rate of pages.

If any JS error occurs in a PV cycle, this PV cycle is an error sample.

Error rate = Error samples/Total samples

Besides automatically reported JS errors, page errors also include errors reported by manually calling the error() method.

Access speed

In ARMS, the access speed indicates the first paint time of the page by default.

In performance speed statistics, all data is calculated based on Navigation Timing APIs defined in W3C standards.


Image source:

Field description

Time consumed in each phase

Reported field Description Calculation Note
dns DNS resolution time domainLookupEnd - domainLookupStart
tcp TCP connection time connectEnd - connectStart
ssl SSL secure connection time connectEnd - secureConnectionStart This field is valid only for HTTPS.
ttfb Time to First Byte (TTFB), that is, network request time responseStart - requestStart TTFB can be calculated in more than one ways. ARMS uses the calculation method defined in Google Development Definitions.
trans Data transmission time responseEnd - responseStart
dom DOM resolution time domInteractive - responseEnd
res Resource loading time loadEventStart - domContentLoadedEventEnd This field indicates synchronously loaded resources on the page.

Key performance metrics

Reported field Description Calculation Note
firstbyte First package time responseStart - domainLookupStart
fpt First Paint Time/blank screen time responseEnd - fetchStart The gap between the beginning of the request and the point when the browser starts to resolve the first batch of HTML bytes
tti Time to Interact domInteractive - fetchStart After the browser completes HTML resolution and DOM creation, it starts to load resources.
ready HTML loading completion time, that is, DOM ready time domContentLoadEventEnd - fetchStart If JSs are executed simultaneously on the page, the simultaneous JS execution time equals to “ready” minus “tti”.
load Fully loaded time of the page loadEventStart - fetchStart load = First paint time + DOM resolution time + Simultaneous JS execution time + Resource loading time

API success rate

API success rate = Successful API calling samples/Total samples

Samples for calculating the API success rate include not only automatically reported AJAX requests, but also data reported by manually calling the api() method.