Understand the idea.
Observe or intervene?
An observational study records conditions that already exist. Researchers might record students' usual sleep and screen time without changing either. Taking measurements does not itself create an experiment: look for an assigned change, not the word 'test'.
An experiment deliberately assigns or changes a condition to compare results. Testing toy cars on different assigned ramp surfaces is experimental. Identify the treatment being changed and the outcome measured so that the purpose of the comparison is clear.
Make comparisons fair
For a fair treatment comparison, keep other relevant conditions similar. Randomly allocating subjects helps prevent one treatment group systematically starting with different characteristics. Use repeated plants, cars or measurements to reveal ordinary variation rather than relying on a single result.
An association from an observational study does not by itself show cause. Sleep and screen time might also relate to health or school workload. Consider these alternative explanations before claiming that changing one variable must change the other.
See the method in action.
Worked example 1
Students report their usual screen time and sleep. No routine changes. Classify the study.
Worked solution
- 1
Find what the researchers do.
Students report their existing screen-time and sleep habits.
- 2
Check whether a treatment is assigned.
Nobody changes or allocates a routine, so the study is observational.
- 3
Limit the conclusion appropriately.
A relationship between reported habits could involve other factors and does not alone establish cause.
Observational.
Worked example 2
Twenty seedlings are randomly split into fertiliser and no-fertiliser groups; water, light and pots are similar.
Worked solution
- 1
Identify the assigned treatment.
The researchers deliberately give fertiliser to one seedling group and none to the other, making this an experiment.
- 2
Check how the groups are compared.
Random splitting helps balance existing seedling differences; similar water, light and pots reduce competing explanations.
- 3
Explain why several seedlings are used.
The twenty seedlings provide repeated observations, showing natural variation within each treatment instead of relying on one plant.
An experiment comparing fertiliser treatments.
Try it yourself.
Check your understanding
A weather station records natural temperatures. Which type of study?
Show answer & reasoning
Observational.
The weather station records naturally occurring temperatures without assigning the weather. Its data collection is observational.
Keep practising
6 worksheet questions · 11 marks
Read the questions online
- A teacher records noise in existing normal lessons. Classify the study.1 mark
- Identical cars are randomly assigned two ramp surfaces. Classify the study.1 mark
- Two different classes use different revision methods and earn different scores. Why is cause not established by this alone?2 marks
- Name two conditions to keep similar when testing two paper-plane designs.2 marks
- A survey links cycling with reported fitness. Write a cautious conclusion.2 marks
- Design a fair battery-brand comparison using torches.3 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- A teacher records noise in existing normal lessons. Classify the study.1 mark
No noise treatment is assigned.
Observational. - Identical cars are randomly assigned two ramp surfaces. Classify the study.1 mark
The surface is deliberately assigned.
Experimental. - Two different classes use different revision methods and earn different scores. Why is cause not established by this alone?2 marks
Other differences could explain the scores.
The classes may differ in prior knowledge, teachers or other conditions. - Name two conditions to keep similar when testing two paper-plane designs.2 marks
These could affect distance independently of design.
For example, paper type and launch method. - A survey links cycling with reported fitness. Write a cautious conclusion.2 marks
Other activity and health differences may contribute.
Cycling and reported fitness are associated in this sample; cause is not established. - Design a fair battery-brand comparison using torches.3 marks
Separate brand from torch/use effects and observe variation.
Randomly allocate batteries to identical torches, keep operation similar, repeat per brand and compare run times.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum
Population, census and sample; observational versus experimental data; random and non-random sampling; primary/secondary data; reviewing numerical displays and summary statistics; comparing samples and sample sizes; bias, variation and uncertainty; planning, conducting and reporting a fair investigation.
Official curriculum source ↗