A reliable method
Use comparison groups, random assignment and replication. A paired design measures the same participant twice or compares meaningfully matched participants, reducing some subject-to-subject variation.
The common trap
A treatment effect cannot be separated from another variable if the two always change together. Blinding can reduce expectation effects, but blinding is not a substitute for random assignment.
Make the connection
Random assignment supports a causal comparison within the experimental setting. Random sampling supports generalizing to the sampled population. Assess these two permissions separately.
Two random steps grant different permissions
Forty volunteers randomly assigned to tutoring A or B support a causal comparison in that experimental setting, but they are not automatically representative of all students. A random sample whose existing tutoring choices are merely recorded supports population description but may retain confounding. Identify the random step before writing a conclusion.
A fair comparison needs an implementable protocol
If A occurs only in morning classes and B only in evening classes, treatment and time are confounded. Instead, randomly assign A and B within each time block and assess students with the same test. Replication reduces random uncertainty. Assignments should be recorded before outcomes are measured, so the result cannot influence who receives each treatment. Blinding assessors can limit expectations, but cannot replace the randomization.
Transfer check
For a before-and-after training study, compute one after-minus-before difference per participant. Explain why treating the two columns as independent loses the pairing. Then identify a remaining threat: without a randomized comparison group, changes could reflect practice, maturation or another time-varying cause.
24 volunteers are randomly assigned to a new tutoring method or the usual method. What does random assignment primarily support?
- Treatments, rather than population members, are assigned randomly.
- Assignment helps distribute confounders between groups over repetitions.
- This supports a causal comparison within the study setting.
Compare the other answer choices
Random assignment does not create a global probability sample.
Randomization and comparison serve different design purposes.
Random assignment balances groups in expectation, not perfectly in every realization.
Try it, then explain it.
Complete a short set without looking at the example. After each answer, explain which operation preserved the relationship and why the result makes sense.
Sources & authorship
Original Slope explanations and examples. References support the concepts; they are not the source of copied questions. Independent educator review is pending.
AP Statistics 2026–27 course framework