Carry out a practical investigation in a biological context, with guidance
What this domain tests
This domain assesses how you plan, carry out, process and evaluate a practical biological investigation, with guidance. The exam looks for valid variables, reliable sampling, accurate measurement, and reasoned evaluation rather than a perfect result.
Core rules and concepts
- The independent variable is what you change; the dependent variable is what you measure; control variables are kept constant.
- Validity means the investigation measures what it claims to measure. It requires a control and only one independent variable.
- Reliability means consistency. Increase it with replication and averaging, not by changing the method mid-run.
- Accuracy is closeness to the true value; improve it with calibrated instruments and correct technique. Precision is the repeatability of a measurement, such as the same decimal place.
- Sampling must be random or systematic and large enough. Avoid choosing only convenient organisms.
- Ethical practice: minimise harm, use humane handling, and follow animal ethics requirements.
Worked example
Investigate light intensity and photosynthesis in Elodea. Change lamp distance (10, 20, 30, 40 cm); count oxygen bubbles per minute; control temperature, CO2 concentration, plant material and time. Repeat each distance three times and average. Rate falls as distance increases until another factor limits photosynthesis. Evaluate: bubble counting is imprecise; a dissolved oxygen probe would be more accurate; heat from the lamp could change temperature, so use a heat filter.
Traps that cost marks
Confusing reliability with validity; omitting a control; not repeating measurements; failing to control lamp heat; concluding more than the data support.