← Module 5: Scientific Investigations
Inquiry Question 2: How does the design of a valid experimental investigation allow for the analysis of first-hand data?
Plan investigations to ensure that they are valid and reliable, including the use of an appropriate experimental design with consideration of independent, dependent and controlled variables
A focused answer to the HSC Investigating Science Module 5 dot point on variables and experimental design. Covers independent, dependent and controlled variables, control groups, sample size, and worked HSC past exam questions.
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What this dot point is asking
NESA wants you to identify the independent, dependent and controlled variables for any given investigation, explain the role of a control group, and design an experiment that produces valid first-hand data. This is fundamental and appears in nearly every Investigating Science paper.
The answer
A scientific investigation tests a hypothesis by changing one thing, measuring the result, and holding everything else constant. Three types of variables structure this design.
Independent variable
The variable that the researcher deliberately changes to test its effect. There is normally one independent variable per experiment, set at multiple levels (e.g. 0, 10, 20, 30 degrees Celsius).
Dependent variable
The variable that the researcher measures to see how it responds. It depends on the independent variable.
Example. In a fertiliser experiment, the independent variable is fertiliser concentration (mg/L) and the dependent variable is tomato yield (kg per plant).
Controlled variables
The variables held constant across all treatment groups to prevent them from confounding the result. In the fertiliser experiment, controlled variables include soil type, watering schedule, sunlight, plant variety and starting plant size.
Control group
A group treated identically to the experimental groups except that it receives no treatment (or a placebo). The control group establishes the baseline against which treatment effects are compared.
Example. A control group of tomato plants given water but no fertiliser. The difference in yield between treated and control plants is attributed to the fertiliser, provided controlled variables are properly held constant.
Replication and sample size
Repeating the experiment, or running multiple individuals per condition, accounts for biological variation and measurement error. NESA expects students to identify a minimum number of replicates (often 5 to 10 per condition) and to repeat the experiment at least three times.
Randomisation and blinding
In human or animal studies, randomisation assigns subjects to treatment groups by chance to reduce selection bias. Blinding prevents the researcher or the subject from knowing the treatment assignment. Double-blind designs are the gold standard in clinical trials.
Common designs
- Comparative design
- Two or more groups treated differently and compared. Most school experiments fit here.
- Time-series design
- One subject measured repeatedly over time as the independent variable changes.
- Factorial design
- Multiple independent variables changed simultaneously to detect interactions (e.g. temperature and pH on enzyme activity).
Past exam questions, worked
Real questions from past NESA papers on this dot point, with our answer explainer.
2023 HSC5 marksA researcher investigated whether caffeine affects the heart rate of Daphnia. Outline the experimental design, identifying the independent, dependent and controlled variables and the role of the control group.Show worked answer →
A 5-mark answer needs the three variable types, a control group, and a clear method.
- Independent variable
- The caffeine concentration applied to the Daphnia (for example, 0, 0.1, 0.5, 1.0 per cent). This is what the researcher deliberately changes.
- Dependent variable
- The heart rate of Daphnia in beats per minute, measured under a microscope.
- Controlled variables
- Temperature of the water (around 18 degrees Celsius), light intensity, age and size of Daphnia, the time of acclimatisation before measurement, the observer doing the counts. These are held constant.
- Control group
- Daphnia placed in pond water with zero caffeine, treated identically in every other way. The control establishes the baseline heart rate against which treatment groups are compared.
- Method
- Acclimatise individual Daphnia in test caffeine concentrations for 5 minutes, then count heart rate for 15 seconds three times per individual, and use at least 10 Daphnia per concentration.
Markers reward correct identification of all three variable types, the role of the control group, and replication.
2021 HSC4 marksExplain why a control group is essential in an investigation of the effect of a new fertiliser on tomato yield.Show worked answer →
A 4-mark answer needs the function of the control, what it isolates, and the consequence of omission.
- A control group
- A group treated identically to the experimental groups in every respect except the independent variable (the fertiliser). It receives no fertiliser, or a placebo solution of equal volume.
- What it isolates
- The control allows the researcher to attribute any difference in tomato yield specifically to the fertiliser, rather than to confounding variables such as soil quality, watering schedule, sunlight, plant variety or seasonal effect.
- Why it is essential
- Without a control, observed tomato yield cannot be compared to a baseline. Any high yield could be due to favourable weather or pest absence, not the fertiliser. The control quantifies what happens in the absence of the treatment.
- Consequence of omission
- Without a control, the experiment cannot conclude that the fertiliser caused the result. Correlation without comparison is not evidence of effect.
Markers reward the definition, the isolation of confounders and an explicit reason why the experiment would fail without a control.
Related dot points
- Develop and evaluate questions and hypotheses for scientific investigation
A focused answer to the HSC Investigating Science Module 5 dot point on inquiry questions and hypotheses. Covers what makes a question testable, the difference between a hypothesis and a prediction, falsifiability, and worked HSC past exam questions.
- Evaluate scientific investigations and findings in terms of reliability, validity, accuracy and precision of data
A focused answer to the HSC Investigating Science Module 5 dot point on reliability, validity, accuracy and precision. The four concepts every Investigating Science student must distinguish, with worked HSC past exam questions.
- Process, analyse and interpret quantitative and qualitative data, including identifying and accounting for sources of error and uncertainty
A focused answer to the HSC Investigating Science Module 5 dot point on data analysis. Covers means and ranges, error bars, significant figures, random vs systematic error, outliers, and worked HSC past exam questions.