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Glossary/Input/Reaction time

Reaction time

Reaction time is how long a person takes to respond to a stimulus; an online test measures that plus the delay the screen, mouse and browser add.

Also known as human reaction time, reaction time test, visual reaction time

Checked by a SystemCheck tool

The reaction time test checks this in your browser, on your own device. The paragraphs below say what it reads and what a browser cannot see.

A reaction time test shows a change and times your response. On a computer the span it times includes the setup as well as you: the screen must draw the change, the mouse must report the click, and the browser must stamp the event. Two results from different sites or setups are therefore not the same measurement, and a fast monitor or mouse shortens what the setup adds without changing your own reaction.

The reaction time test runs 5 trials, each waiting a random 1.5 to 4 seconds before the colour changes. It times from the animation frame the change was drawn in to the click event's own timestamp. A click before the change is early, one within 100 ms of it is a guess, our line, and neither counts. The figure is the median, because one fast trial can be luck.

The test then subtracts the setup's estimated share, as ranges, from intervals it measures on the page: half to one and a half refresh intervals for the screen, 3.5 to 10.4 ms at 144 Hz, and up to one report interval for the mouse. Still inside the figure are the monitor's processing and pixel response, the mouse's switch, and the path to the browser. It does not rank you against other people, because there is no population behind it to rank against.

Covered in full

This page defines the term. The pages below explain it, diagnose it and measure it.

Related terms

Input lagInput lag is the time from a physical input, such as a click or key press, to the screen showing its result; it adds up across every part of the chain.Read moreRefresh rateRefresh rate is how many times a second a display draws a new image, in hertz; the interval between images is 1000 divided by the rate, in milliseconds.Read morePolling ratePolling rate is how often a mouse, keyboard or controller reports its state to the computer, in hertz; the interval is 1000 divided by the rate in ms.Read moreMouse DPI (CPI)Mouse DPI, more accurately CPI, is how many movement counts a mouse reports for each inch it travels across the desk.Read more
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