How large is the average delay from key-presses

Question

I am currently helping someone with a reaction time experiment. For this experiment reaction times on the keyboard are measured. For this experiment it might be important to know, how much error could be introduced because of the delay between the key-press and the processing in the software.

Here are some factors that I found out using google already:

• The USB-bus is polled at 125Hz at minimum and 1000Hz at maximum (depending on settings, see this link).
• There might be some additional keyboard buffers in Windows that might delay the keypresses further, but I do not know about the logic behind those.

Unfortunately it is not possible to control the low level logic of the experiment. The experiment is written in E-Prime a software that is often used for this kind of experiments. However the company that offers E-Prime also offers additional hardware, that they advertise for precise reaction-timing. Hence they seem to be aware about this effect (but do not tell how large it is).

Unfortunately it is necessary to use a standart keyboard, so I need to provide ways to reduce the latency.

Answer 1

any latency from key presses can be attributed to the debounce routine (i usually use 30ms to be safe) and not to the processing algorithms themselves (unless you are only evaluating the first press).

Answer 2

If you are running an experiment where millisecond timing is important you may want to use http://www.blackboxtoolkit.com/ to find sources of error.

Your needs also depend on the nature of your study. I've run RT experiments in Eprime with a keyboard. Since any error should be consistent on average across participants, for some designs it is not a big problem. If you need to sync up the data though with something else (like Eye tracking or EEG) or want to draw conclusions about RT where specific magnitude is important then E-Primes serial resp box (or another brand, though I have had compatibility issues in the past with other brand boxes and eprime) is a must.