Measuring and Investigating Reaction Time

What reaction time measures

Reaction time is the time between a stimulus occurring and a person responding to it. When you catch a falling ruler, its movement is the stimulus. Receptors detect the movement, the nervous system processes the information, and muscles contract to close your fingers. Catching the ruler is a conscious response, rather than an automatic protective reflex.

Reaction times vary between people and with the conditions of the test. Typical human reaction times range from 0.3 to 0.9 seconds. This is a typical range, not a boundary that every measurement must fall within: results depend on the task and the person. Factors such as age, stress and caffeine intake can affect reaction time.

Measuring reaction time with a falling ruler

A ruler-drop test measures how far a ruler falls before someone catches it. A longer delay allows the ruler to fall further, so a smaller distance means a faster reaction. The distance is then converted into a time.

You need a partner, a metre ruler, a table and chair, a results table and a distance–time conversion table. A metre ruler gives more opportunity to catch the ruler than a short ruler.

1. The participant sits upright, looking across the room, with the forearm of their dominant arm supported on the table and their hand overhanging its edge.
2. Their partner holds the ruler vertically, with the 0 cm end downwards, between the participant’s thumb and first finger. Align the zero mark with the top of the thumb, so the reading will represent the distance fallen.
3. Tell the participant to prepare. Release the ruler without a countdown or warning of the exact moment; do not push it downwards.
4. The participant closes their thumb and first finger as quickly as possible when the ruler drops.
5. Once the ruler is caught, read the scale level with the top of the thumb and record the distance in centimetres.
6. Allow a short rest and repeat several times. Partners can then swap roles, with each using their own dominant hand.

Keep the falling ruler away from faces and clear the space below the hand. Participants should take part willingly and be able to stop if uncomfortable. If the ruler is missed, record a missed catch rather than inventing a distance.

Turning the ruler reading into a time

A ruler reading is a distance, not a reaction time. Use a distance–time conversion table to find the corresponding time in seconds. For example, the following pairs show how ruler measurements can be converted:

The 19 cm catch represents the fastest response of these three. The relationship is not directly proportional because a falling ruler speeds up: doubling the distance does not double the time. Convert each reading before calculating a mean reaction time.

A computer-based test provides another method. A visual stimulus appears on screen, and the participant presses a key or clicks as soon as they detect it. The programme records the interval directly, often in milliseconds, where $1000\text{ ms}=1\text{ s}$. This avoids manually reading a ruler scale, although the device and the participant’s attention can still affect the result.

Planning an investigation into distraction

Required practical activity 8 investigates the effect of a factor on human reaction time. One manageable question is: Does background music change reaction time compared with a quiet room?

A reasonable hypothesis is that music may distract the participant, increasing their reaction time. This is a prediction to test, not a result to assume.

The independent variable is the background condition: quiet or music. The dependent variable is reaction time, obtained from the ruler-drop distance. Keep the following conditions constant:

• The participant and their dominant hand, so differences between people or hands do not explain the change.
• The ruler, its vertical orientation and its starting position.
• The seated posture, instructions and person releasing the ruler.
• The number of trials and the rest between trials.
• The music track and volume during the music condition.

Use music at a comfortable, safe volume. There is no need to expose participants to very loud sound or give them substances to investigate reaction time.

Give each participant the same short practice period. Then collect, for example, ten catches in quiet conditions and ten with music. Each participant completes both conditions, allowing a comparison within the same person. Where several people take part, have some complete the quiet condition first and others the music condition first. This helps prevent practice or tiredness from being confused with the effect of music.

Prepare the results table before starting. For each numbered attempt, record separate columns for ruler distance in cm and converted reaction time in s for each participant under each condition. This keeps the repeated measurements and the comparisons clearly organised.

Drawing a conclusion and judging the method

Calculate a mean reaction time for each participant in each condition:

$$\text{mean reaction time}=\frac{\text{sum of reaction times}}{\text{number of reaction times}}$$

A higher mean with music would support the prediction that this distraction slowed responses. A lower mean would suggest faster responses, while similar means would provide little evidence of an effect under the conditions tested. Use the results to decide whether the hypothesis is supported, rather than treating the prediction as proof.

Look at the spread of the repeated results as well as their means. A small difference between means is less convincing if the individual readings vary greatly. More participants help you judge whether a pattern extends beyond one person.

Possible problems include anticipating the drop, inconsistent zero-mark alignment, misreading the scale and improvement through practice. Repeats reduce the influence of random variation, but they cannot fix a starting position that is wrong every time. Investigate unusually different readings and repeat a trial where possible; only exclude a result with a justified reason.

Reaction time also matters in road safety. A driver continues travelling between noticing a hazard and starting to brake. At the same speed, a longer reaction time therefore means a greater thinking distance.

Core recall

• Reaction time: interval between a stimulus and a response.
• Typical human values: 0.3–0.9 s; people and tasks vary.
• Longer reaction time increases a driver’s thinking distance at the same speed.

Measurement methods

Ruler drop: dominant forearm supported, hand over table edge; ruler vertical with 0 cm downwards and aligned with the top of the thumb. Drop without warning of the exact moment, catch, and read at the top of the thumb.

Smaller distance → shorter reaction time. Convert each distance using a distance–time table. Rest and repeat.