Experiments and Random Assignment
A free Statistics and Data Analysis lesson from the “Collecting Data” unit, with a worked example and practice problems including step-by-step solutions.
An experiment deliberately applies treatments to compare outcomes. Random assignment helps make treatment groups comparable, supporting cause-and-effect conclusions when the design is sound. This lesson builds the habit of reading the context first, choosing the right statistical tool, calculating carefully, and then writing what the result means. By the end, students should be able to do the computation and explain why that computation answers the question.
What you'll learn
- Identify treatments and experimental units
- Explain random assignment
- Connect experiments to cause-and-effect conclusions
Worked example
Problem. A teacher randomly assigns students to use either Study App A or Study App B, then compares quiz scores. What is the role of random assignment?
- Random assignment spreads other factors across groups.
- That makes the treatment comparison fairer.
Answer: make the treatment groups comparable
Lesson preview: this page shows 5 practice items. The interactive activity contains 28 practice items and 15 quiz items (multiple choice). Counts include repeated prompts and variants.
Practice problems
1. Practice case A: In an experiment comparing two fertilizer types, what is the treatment?
Choices: the condition assigned to a group · the population being sampled · the final p-value · the survey wording
Show solution
- A treatment is what researchers deliberately apply.
- Groups are compared after receiving different treatments.
Answer: the condition assigned to a group
2. Practice case B: In an agriculture test assigning plots to fertilizers, what are the experimental units?
Choices: the treatment names · the margin of error · the null hypothesis · the individuals or objects receiving treatments
Show solution
- Experimental units are assigned to treatments.
- They are the objects or people being studied.
Answer: the individuals or objects receiving treatments
3. Practice case C: Why should treatment groups be assigned by chance?
Choices: to make the study observational · to avoid measuring outcomes · to make groups similar before comparing outcomes · to guarantee a larger sample
Show solution
- Random assignment balances other factors across groups.
- That supports a fair comparison.
Answer: to make groups similar before comparing outcomes
4. Practice case D: A test gives one class a new review method and another class the current method. What is the current-method group?
Choices: sample statistic · control group · population · voluntary response group
Show solution
- A control group provides a comparison.
- It helps judge whether the new treatment changed the outcome.
Answer: control group
5. Practice case E: A randomized experiment finds that reminders assigned to one group increased submissions. What kind of conclusion can it support?
Choices: cause and effect for the treatment comparison · only association with no causal support · the exact population parameter · that the sample was randomly selected
Show solution
- Random assignment is the key design feature for causation.
- It supports cause-and-effect claims about the treatment.
Answer: cause and effect for the treatment comparison
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