You are viewing a free preview of this lesson.
Subscribe to unlock all 8 lessons in this course and every other course on LearningBro.
This lesson is mapped to AQA 7402 Required Practical 10 — investigation of a factor affecting reflex / response in an organism (refer to the official AQA specification document for exact wording). It is examined directly on Paper 3 (Section A practical-skills questions) and contributes to the Practical Endorsement (CPAC criteria 1–5). This lesson uses the reaction-time ruler-drop test as the canonical implementation, with muscle fatigue as a secondary investigation, and treats the experiment with the analytical rigour expected at A-Level — careful experimental design, controlled variables, statistical analysis, and explicit consideration of sources of error.
The lesson is not merely a method recipe. It is an opportunity to consolidate the content of the entire course: every step of a reaction-time response uses the structures and processes covered in lessons 0–6, and the data-analysis skills here are transferable to every required practical in 7402.
Key Definition: A reaction time is the latency between presentation of a stimulus and execution of a motor response. For visual ruler-drop, a typical adult value is 150–250 ms.
The ruler-drop test is the AQA-standard practical for measuring reaction time. It exploits the kinematics of free fall to convert a length measurement (the distance the ruler falls before being caught) into a time measurement (the latency between drop and catch).
The ruler is in free fall under gravity. From the kinematic equation s = ½gt², solving for time:
t = √(2s / g)
where s is the drop distance in metres and g = 9.81 m s⁻². Therefore:
t (s) = √(2 × d(m) / 9.81)
Working in centimetres for the measured d:
t (ms) ≈ 14.3 × √d(cm) (good approximation)
| Drop distance | Reaction time |
|---|---|
| 5 cm | ~100 ms (elite athletes) |
| 10 cm | ~143 ms |
| 15 cm | ~175 ms |
| 20 cm | ~202 ms (typical adult) |
| 25 cm | ~226 ms |
| 30 cm | ~247 ms (slower than average) |
The non-linear scale matters: a 5 cm change at low distances corresponds to a much smaller time difference than a 5 cm change at high distances. Always convert to time before averaging or applying statistics.
The AQA practical asks you to investigate a factor that affects reflex / response. Choose ONE independent variable. Examples:
For this discussion we will use caffeine as the worked example because of its direct synoptic link to lesson 2 (adenosine receptor antagonism).
| Variable | Reason | Control |
|---|---|---|
| Handedness | Dominant hand is faster | Use dominant hand only |
| Time of day | Diurnal variation in alertness | Test all participants at same time of day |
| Recent caffeine intake | Long-acting (half-life ~5 h) | 12 h abstention before "no caffeine" trial |
| Recent food / sleep | Both affect alertness | 1 h post-meal, normal sleep night before |
| Distraction | Affects all reaction times | Quiet environment, eyes on ruler |
| Rest between trials | Prevents fatigue | ~15 s between trials |
| Experimenter | Different experimenters release at different speeds | Same experimenter for all trials of one participant |
| Practice / learning effect | Subjects improve with practice | Discard first trial; "practice run" before recording |
For each condition, calculate:
A reasonable cut-off for outliers is "mean ± 2 standard deviations". Any single trial more than 2 SD from the participant's own mean is likely a non-physiological event (lapse, anticipation, dropped ruler) and should be flagged. Decide before the experiment whether you will discard outliers — post hoc selection of which trials to keep is a serious form of bias.
Was the difference between conditions due to the independent variable, or chance?
Choice of test depends on data type and design:
| Design | Test | When |
|---|---|---|
| Two independent groups, normally-distributed data | Independent-samples (Student's) t-test | Comparing caffeine-drinkers vs non-drinkers |
| Two paired measurements, normally-distributed differences | Paired-samples t-test | Same participants before vs after caffeine |
| Two groups, non-normal data or small n | Mann-Whitney U (independent) or Wilcoxon signed-rank (paired) | Use if Shapiro-Wilk test rejects normality |
| Multiple groups | One-way ANOVA | Comparing low / medium / high caffeine doses |
The test produces a p-value — the probability that the observed difference (or larger) would arise by chance if the null hypothesis (no effect) were true. Conventional threshold: p < 0.05 is considered statistically significant.
| Reality: H₀ true | Reality: H₀ false | |
|---|---|---|
| Test concludes H₀ true | Correct (1−α) | Type II error (β) — missed a real effect |
| Test concludes H₀ false | Type I error (α) — false positive | Correct (1−β) — power |
Power analysis before the experiment uses an estimate of the expected effect size and the desired power to calculate the minimum sample size needed. A typical reaction-time caffeine effect is ~10–20 ms with within-subject SD of ~30 ms — power analysis suggests n ≥ 30 paired participants for 80% power at α = 0.05.
Failing to perform a power analysis risks Type II errors: a null result might mean no effect or simply an underpowered study. Many A-Level practical investigations are underpowered and over-interpret null results — an A* response recognises this.
Every measurement has error. The A-Level skill is to identify them, classify them as random (cancel on repeated measurement) or systematic (do not cancel), and propose mitigations.
Reaction time depends on age, fitness, training, time of day, fatigue, mood, and motivation. This is not strictly "error" — it is genuine biological variation — but it inflates between-participant variance and reduces statistical power. The paired-design recommendation above addresses exactly this issue.
Caffeine is a competitive antagonist at adenosine A1 and A2A receptors. Adenosine accumulates in the brain during wakefulness and promotes sleepiness by reducing neural firing via adenosine receptors on excitatory synapses. By blocking adenosine receptors, caffeine prevents this "brake" and maintains alertness. The molecular mechanism is therefore a synaptic-level intervention that reduces inhibitory tone, allowing slightly faster propagation of signals through the reflex chain.
Quantitatively, caffeine typically reduces simple reaction time by 10–30 ms — small in absolute terms but ~5–10% of baseline, and biologically meaningful in performance contexts (driving, sport, surgery).
The ruler-drop test measures a voluntary reaction, not strictly a reflex. The latency partitions roughly as:
Total: ~150–200 ms. This is consistent with measured values and validates the framework of the whole course.
The sampling design principles in RP10 — sample size, randomisation, paired vs independent design, statistical testing — transfer directly to RP11 (ecological sampling). The same logic underpins every quantitative biology investigation.
This lesson is the anchor for AQA 7402 Required Practical 10 — investigation of a factor affecting reflex / response (refer to the official AQA specification document for exact wording). It contributes to the Practical Endorsement and is examined directly on Paper 3.
Practical-skills questions split AO marks predictably:
Subscribe to continue reading
Get full access to this lesson and all 8 lessons in this course.