Disease and response · GCSE Biology
Reaction time required practical
Plan the GCSE Biology reaction-time investigation: ruler-drop or computer test, caffeine or tiredness as the independent variable, reaction time as the dependent variable, controls, mean and evaluation.
Independent variable: caffeine or tiredness. Dependent variable: reaction time in milliseconds or seconds. Controls: same person, same hand, same drop height, same instructions. Calculate a mean and discard anomalies.
The important bits
What you need to know
- 1
Aim: investigate how a factor such as caffeine intake or tiredness affects human reaction time.
- 2
Ruler-drop method: partner holds a metre ruler vertically; the subject catches it as soon as they see it fall. Reaction time t = √(2d/g), where d is the distance fallen in metres and g ≈ 9.8 m/s². Alternatively, read time directly from a computer reaction test.
- 3
Independent variable examples: volume of a caffeinated drink versus decaffeinated control; reaction time at the start of the day versus after a night of poor sleep; before and after exercise (if linked to tiredness).
- 4
Dependent variable: reaction time — the time between the stimulus (ruler drops or screen changes) and the response (catch or button press). Record in seconds or milliseconds.
- 5
Control variables: same participant (or clearly separate groups if comparing people), same dominant catching hand, same ruler drop height or same computer test, same room and lighting, no distractions, and a fixed gap between trials.
- 6
Repeat each condition at least five times and calculate the mean reaction time. Exclude clear anomalies (ruler knocked sideways, participant not looking) before averaging.
- 7
Caffeine is a stimulant: it may decrease reaction time (faster responses) by increasing alertness. Tiredness or alcohol increases reaction time (slower responses) — relevant to stopping distances in driving.
- 8
Evaluation: ruler-drop depends on catching skill and anticipation; a computer test is more consistent. Practice trials reduce learning effects. Ethical note: inform consent, no coercion, and safe caffeine doses for age.
Quotations worth analysing
Short evidence. Real method.
“reaction time = √(2 × distance fallen / g)”
Convert centimetres to metres before substituting. A shorter distance means a faster reaction. Show the square root step.
“Independent variable: factor you change (caffeine or tiredness). Dependent variable: reaction time.”
State both in exam language. “How fast someone is” is not precise enough for method marks.
“Calculate a mean and identify anomalies before drawing conclusions.”
One fast catch after five slow ones should not alone prove caffeine works. Means and repeats are how you defend the pattern.
Go deeper
Ruler-drop versus computer: which story fits your answer?
In the ruler method, the partner rests the ruler between the subject’s thumb and finger without touching, then releases without warning. The subject catches it; the distance above the thumb is read in centimetres and converted to time. Higher on the ruler means a slower reaction. The formula assumes the ruler falls freely from rest — hold it vertically and avoid spinning it. A computer test records time from a colour change to a key press; there is no square root, but the same IV/DV language applies. If the question says “required practical”, name the method you were taught. For six marks, include repeats, mean, and a control condition (decaffeinated drink matched for volume and temperature, or rested versus sleep-deprived on different days with the same test time).
Go deeper
How do caffeine and tiredness link to the nervous system?
Reaction time measures how quickly a stimulus is detected and a motor response starts — sensory neurone, CNS processing, motor neurone, muscle contraction. Caffeine blocks adenosine receptors in the brain, increasing alertness; many students show slightly shorter reaction times after a moderate caffeinated drink. Tiredness slows CNS processing and is a model for why driving when fatigued increases thinking distance. Alcohol and some drugs have a similar effect and are specification examples. In the write-up, link the result to alertness of the nervous system, not just “they were quicker”. A bar chart of mean reaction time for “no caffeine” versus “caffeine” is clearer than plotting every raw catch distance without converting to time.
Go deeper
What makes a weak investigation, and how do you evaluate it?
Anticipation ruins validity: if the dropper always releases on a count of three, the subject reacts to the pattern, not the fall. Randomise release times or use software. Learning effect: the first five catches are often slower than the next five; include practice trials that you do not plot. Small sample: one person before and after coffee is a case study, not proof for all humans — state that in the evaluation. Caffeine dose, sleep history and food intake are confounding variables; control them or acknowledge them. Improvements: more participants, double-blind decaffeinated control, computer timing, and testing at the same clock time on different days. Compare your mean difference to the spread of the data — if means overlap a lot, the conclusion should be cautious.
See the idea in action
A student catches a ruler at 18 cm after a decaffeinated drink (five trials: 20, 19, 18, 18, 17 cm → mean 18.4 cm = 0.184 m). t = √(2 × 0.184 / 9.8) = 0.19 s (190 ms). After caffeinated coffee, mean catch distance is 14 cm → t = √(2 × 0.14 / 9.8) = 0.17 s (170 ms). Mean reaction time is about 20 ms faster with caffeine. IV: caffeinated versus decaffeinated drink. DV: reaction time from ruler distance. Controls: same hand, same drop height, five repeats, mean calculated, anomalies removed. Conclusion: caffeine may reduce reaction time, supporting increased alertness. Evaluation: only one participant; anticipation could affect the next trial — use random release times.
Exam technique
Turn knowledge into marks
State IV, DV and at least three controls. Show mean calculation and mention repeats. Link caffeine to alertness or tiredness to slower responses. Evaluate anticipation, sample size and consistency of the test.
Common mistakes
Do not give these marks away
- 01
Swapping the independent and dependent variables, or calling “reaction time” the factor being changed.
- 02
Using distance fallen as the final answer without converting to time when the question asks for reaction time.
- 03
Drawing a conclusion from a single trial, or forgetting that tiredness increases reaction time rather than decreasing it.
In a reaction-time investigation, what is the dependent variable?
AWhether the participant drank caffeine
BHow many hours they slept
CThe reaction time measured by ruler drop or computer test
DThe height from which the ruler is held
Show the answer
The reaction time measured by ruler drop or computer test. The dependent variable is what you measure. Caffeine or tiredness is the independent variable. Drop height should be controlled, not measured as the DV unless you convert to time.
Quick questions
If this is the bit you searched
What is the independent variable in the reaction-time practical?
The factor you change, such as caffeinated versus decaffeinated drink, or rested versus tired state. Reaction time is the dependent variable.
How do you calculate reaction time from the ruler-drop test?
Measure the distance fallen d in metres, then use t = √(2d/g) with g = 9.8 m/s². Shorter distance means faster reaction.
Why take repeats and calculate a mean?
Single catches vary. A mean smooths random error and makes comparison between conditions fairer. Anomalies can be excluded with justification.
How does tiredness affect reaction time?
Tiredness usually increases reaction time — responses are slower. That is why fatigue increases thinking distance when driving.