Designing and evaluating biological investigations (criterion 3) - TCE Biology (Tasmania)
“Science inquiry skills: identify, research and construct questions, propose hypotheses and predict outcomes; design investigations (procedures, materials, data to collect, risk assessment, research ethics including animal ethics and the rights of living organisms); collect valid and reliable data; represent and analyse data (mean, median, range, trends and relationships; measurement error, accuracy, procedure and sample size as sources of uncertainty); evaluate processes, claims and conclusions”
Criterion 3 tests whether you can plan and evaluate an investigation. Identify the independent variable (what is changed), the dependent variable (what is measured, and how), and controlled variables (kept the same). Write a hypothesis that states the effect of the independent variable on the dependent variable with a clear direction, and justify it with biology. Use a control group where needed, repeat trials and calculate means for reliability, control variables for validity, and consider ethics. Evaluate by naming specific limitations and specific improvements, not "human error".
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What this dot point is asking
Criterion 3 (undertake biological inquiry to generate and evaluate data) is Section A of the TASC exam: 36 marks, about 36 minutes, four to six compulsory questions. The external assessment specifications list research ethics, formulating a hypothesis (worth 3 marks), designing a controlled experiment, and evaluating a method and suggesting improvements, in contexts from the other four criteria. Laboratory safety is not examined in Section A, and data analysis questions use the mean; confidence intervals are not examined.
Variables
- Independent variable (IV): the one factor the investigator deliberately changes.
- Dependent variable (DV): what is measured to see the effect of the IV. State what is measured and how (the DV is catalase activity; the measurement is the volume of oxygen produced in 1 minute).
- Controlled variables: factors kept the same so they cannot affect the DV. Choose ones specific to the organism or reaction (substrate concentration, temperature, pH, light intensity), and be able to explain how each would change the results if not controlled.
TASC's 2025 assessment report noted that many students confused the dependent variable with the method of collecting data, and that broad statements such as "may alter results" or "ensures validity" earned no marks.
Writing a hypothesis
A hypothesis is a precise, testable statement that predicts the effect of the IV on the DV, with direction. A top answer also justifies it with biology.
- Weak: "Temperature will affect catalase." (no direction)
- Strong: "As temperature increases from 10 to 40 degrees Celsius, the volume of oxygen produced by catalase in one minute will increase, because enzyme and substrate molecules collide more often, but above 40 degrees Celsius it will decrease as the enzyme denatures."
TASC's 2025 assessment report noted that many students did not follow the "effect of IV on DV, with direction" formula, writing that the DV would "change" or "increase or decrease".
Designing a controlled experiment
- Change only one variable (the IV), so any change in the DV can be attributed to it.
- Control group: a group that does not receive the treatment (or receives a placebo), used for comparison to show that the effect is caused by the IV. A control group is different from controlled variables.
- Repeat trials and replicates: several trials at each level of the IV, with a mean calculated.
- Sample size: larger samples reduce the effect of individual variation.
- Method detail: materials, quantities, how and when measurements are taken, and a risk assessment.
Validity, reliability and accuracy
| Term | Meaning | How to improve it |
|---|---|---|
| Validity | The investigation tests what it claims to test; only the IV affects the DV | Control variables; use a control group; measure the DV directly |
| Reliability | Results are consistent when repeated | Repeat trials; use replicates and calculate a mean; larger sample size |
| Accuracy | Measurements are close to the true value | Use calibrated, more sensitive equipment |
| Precision | Repeated measurements are close to each other | Use equipment with finer scales; standardise technique |
TASC's 2025 assessment report noted that students were confused about accuracy and precision, so learn the difference.
Processing and analysing data
- Calculate the mean of repeated trials; identify and explain anomalous results.
- Calculate rates (for example, volume of gas per minute) and percentage change.
- Draw graphs with a title, the IV on the x-axis, the DV on the y-axis, labelled axes with units, and an appropriate scale; decide whether to join points or draw a line of best fit.
- Describe trends and relationships, quoting values, and explain them with biology.
- Discuss sources of uncertainty: measurement error, the accuracy of instruments, the procedure and the sample size.
Research ethics
Consider the rights and welfare of living organisms:
- Animals: use the fewest animals needed, minimise pain and distress, and use alternatives where possible.
- Humans: informed consent that explains all risks, including the chance of receiving a placebo; approval from an ethics committee; the right to withdraw; confidentiality. When an effective treatment exists, withholding it from a placebo group can put participants at risk, which is an ethical issue that TASC's 2025 assessment report expected students to discuss scientifically.
Every evaluation mark needs a specific limitation linked to a specific improvement and its effect, for example "the lamp also heated the water, so temperature was not controlled; use a heat shield or water bath to keep temperature constant, improving validity".
Testing antibiotics against bacteria
Agar plates spread with bacteria have paper discs soaked in antibiotics W, X and Y. After 48 hours, clear zones of inhibition form around the discs. There is one plate per bacterium.
- Conclusion: the antibiotic with the largest zone of inhibition is most effective against that bacterium, because it prevented growth over the largest area.
- Reliability: one plate each is not enough; use several replicate plates and calculate a mean zone diameter.
- Accuracy: measure the zone diameter with a ruler across two perpendicular lines rather than estimating by eye.
- Validity: use discs of the same size soaked for the same time in the same concentration, and include a control disc soaked in sterile water.
Marker's note: name the aspect being improved (reliability, validity or accuracy) and explain how the change improves it.
- Hypotheses with no direction
- State whether the DV will increase or decrease.
- Confusing controlled variables with a control group
- Controlled variables are kept constant; a control group lacks the treatment.
- "Human error" as an evaluation
- Name the specific error and a specific fix.
- Mixing up the DV and the measuring method
- The DV is what is affected; the method is how you measure it.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original7 marksA student investigates whether temperature affects the activity of the enzyme catalase by timing how long it takes for discs of filter paper soaked in catalase to rise to the top of hydrogen peroxide at 10, 20, 30, 40 and 50 degrees Celsius. (a) Identify the independent and dependent variables. (b) Write a hypothesis. (c) Identify one controlled variable and explain how it could affect the results if it were not controlled.Show worked answer →
(a) 2 marks. IV: temperature of the hydrogen peroxide. DV: catalase activity, measured as the time taken for the disc to rise (a shorter time means more oxygen produced, so higher activity).
(b) 3 marks. As the temperature increases from 10 to 40 degrees Celsius, the time taken for the disc to rise will decrease (catalase activity will increase), and above 40 degrees Celsius the time will increase because the enzyme begins to denature. The hypothesis names the IV and DV, states the direction of the effect and gives a biological reason.
(c) 2 marks. The concentration of hydrogen peroxide (the substrate). If it were higher in some trials, more substrate would be available to the enzyme, so oxygen would be produced faster and the disc would rise sooner, making activity appear higher at that temperature regardless of the temperature itself.
Original4 marksThe student tested each temperature once. Suggest two specific improvements to the method and explain how each would improve the investigation.Show worked answer →
Two marks for each improvement explained.
Repeat trials. Carry out at least three trials at each temperature and calculate a mean time. This improves reliability, because anomalous results can be identified and their effect reduced.
Control the temperature more precisely. Place the hydrogen peroxide in a thermostatically controlled water bath and allow it to reach the set temperature before adding the disc, checking with a thermometer. This improves validity, because the enzyme is actually at the temperature being tested.
Other accepted improvements: use discs cut to the same size with a hole punch and soaked for the same time (controls the amount of enzyme); use a more precise measure of oxygen, such as collecting the gas in a syringe.