The vocabulary markers look for
| Term | Meaning | How to write it |
|---|
| Aim | What the investigation sets out to find | “To investigate the effect of [independent] on [dependent].” |
| Investigative question | The aim as a question | “What is the effect of … on …?” |
| Hypothesis | A testable prediction linking the two variables | “An increase in X will increase / decrease / have no effect on Y.” |
| Independent variable | What the investigator deliberately changes | Usually the x-axis |
| Dependent variable | What is measured | Usually the y-axis; include how it is measured |
| Controlled (fixed) variables | Factors kept the same in all groups | Name specific ones: temperature, volume, species, age |
| Control (group) | A set-up without the factor being tested, for comparison | “To show that the results are due to X only.” |
Reliability, validity and ethics
- Reliability — can the results be trusted to be repeatable? Improve it by repeating the investigation, using a larger sample, and calculating averages.
- Validity — does the investigation test what it claims to? Improve it by controlling variables, using a control, and measuring accurately.
- Ethics (human or animal studies) — informed consent, confidentiality, no harm, the right to withdraw.
- Accepting or rejecting a hypothesis: “The hypothesis is accepted because the results show that …” — always quote the evidence.
Common mark losses: a hypothesis written as a question; naming “the amount” instead of a specific controlled variable; saying “to make it fair” without explaining; reaching a conclusion that goes beyond the data.
Worked example: light intensity and photosynthesis
Learners placed pondweed at different distances (10, 20, 30, 40 cm) from a lamp and counted the oxygen bubbles released per minute. Each distance was tested three times.
| Distance (cm) | Trial 1 | Trial 2 | Trial 3 | Average |
|---|
| 10 | 42 | 38 | 40 | 40 |
| 20 | 30 | 29 | 31 | 30 |
| 30 | 18 | 22 | 20 | 20 |
| 40 | 11 | 9 | 10 | 10 |
✏️ Model answers
1Independent variable: light intensity (distance from the lamp). Dependent: rate of photosynthesis (number of bubbles per minute).
2Two controlled variables: temperature of the water; the same species and length of pondweed (also CO₂ concentration).
3Hypothesis: an increase in light intensity will increase the rate of photosynthesis.
4Why three trials? To improve reliability — an average reduces the effect of errors.
5Conclusion: as light intensity increases (distance decreases), the rate of photosynthesis increases. The hypothesis is accepted.
6Why place a beaker of water between the lamp and the plant? To absorb heat from the lamp, so that temperature stays constant.
Planning an investigation
A “plan an investigation” question (often 5–7 marks) is marked on a checklist. Include:
- The apparatus and the groups (including a control).
- What will be changed and how (the independent variable, with specific values).
- What will be kept constant (at least two variables).
- What will be measured, how, and for how long.
- Repeats and sample size, for reliability.
- Safety and ethics where relevant.