The idea, step by step
Observe or intervene?
An observational study records existing conditions without assigning a treatment. Recording usual sleep and screen time is observational. An experiment deliberately changes a condition, such as assigning different ramp surfaces to identical toy cars. Measuring something, or calling it a test, does not automatically make a study experimental.
Make comparisons fair
Keep relevant conditions similar apart from the factor being tested. Random allocation can reduce systematic group differences, and repeated measurements reveal ordinary variation. An association in observational data does not by itself establish cause: prior knowledge, health or other factors might explain a difference.
A common trap
Any measured study is an experiment.
Instead: An experiment requires deliberate assignment or change of a condition.
See the thinking
Worked example 1
Students report their usual screen time and sleep. No routine changes. Classify the study.
- The study records existing habits.
- No treatment is assigned.
- A link would not by itself prove a cause.
Worked example 2
Twenty seedlings are randomly split into fertiliser and no-fertiliser groups; water, light and pots are similar.
- Fertiliser is deliberately assigned.
- Other growing conditions are controlled.
- Repeated plants help reveal variation.
Check your understanding
A weather station records natural temperatures. Which type of study?
Show answer
Observational.
The station records rather than assigns conditions.
Your turn
6 questions · 11 marks · Work at your own pace.
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 ↗