Human Reaction Times

A typical person takes about 0.20–0.25 s to respond to a stimulus. This is only a fraction of a second, but an object can still move a noticeable distance during it. Reaction times vary between people and between repeated attempts by the same person, so a single result should not be treated as a precise measurement of someone's usual reaction time.

Measuring reaction time with a ruler

A simple laboratory method uses a metre ruler and a partner:

1. The person being tested holds their thumb and index finger apart, ready to catch the ruler.
2. Their partner holds the ruler vertically, with its lowest numbers at the bottom and the zero mark level with the gap between the open thumb and finger.
3. Without warning, the partner releases the ruler. The ruler beginning to fall is the stimulus.
4. The person closes their fingers to catch it. This is the response.
5. Read the distance fallen from the ruler at the point where it was caught.

A short fall distance means that the ruler was caught quickly; a long fall distance means that the reaction took longer. The release must be unexpected, because a countdown could allow the person to anticipate the stimulus rather than react to it.

Converting fall distance into reaction time

As the ruler falls from rest, its distance of fall is related to time by

$d=\frac{1}{2}gt^2$

Rearranging gives

$t=\sqrt{\frac{2d}{g}}$

Here, $t$ is reaction time in seconds, $d$ is the distance fallen in metres and $g$ is gravitational field strength. For GCSE estimates near Earth's surface, use $g=10\ \text{m/s}^2$ unless another value is supplied.

The square-root relationship means that reaction time is not directly proportional to fall distance. If the ruler falls four times as far, the calculated reaction time is twice as long.

Measuring reaction time electronically

Reaction time can also be measured using a computer or another electronic timer. The program presents an unexpected stimulus, such as a visual signal, and the participant responds by pressing a key or button as quickly as possible. The program records the interval between presenting the stimulus and detecting the response, commonly in milliseconds.

This method measures time directly rather than estimating it from a ruler's fall distance. Automatic timing also avoids uncertainties caused by reading the ruler or converting distance into time. However, results may still vary because of the participant, anticipation or the response device, so several trials are needed.

The stimulus and response must be stated when describing either method. In a ruler test they are the ruler beginning to fall and the fingers closing; in a computer test they might be a signal appearing and a key being pressed.

Repeats, a mean and a fair comparison

Repeat either test several times because concentration and readiness can vary from trial to trial. Record each result, identify any anomalous result and calculate a mean from the appropriate results.

For a fair comparison, change only the factor being investigated and keep the rest of the procedure consistent. In a ruler test, use the same person, hand and ruler, begin from the same zero position and release the ruler vertically without warning. In an electronic test, use the same device, stimulus and response control. Repeats improve reliability by reducing the influence of random variation, although they do not correct a systematic problem with the apparatus or method.