Separate Random Assignment from Random Sampling
Random assignment distributes participants among treatment groups to reduce systematic preexisting differences.
About 25 minutes, plus any extended practice
Supplied materials
- Paper or a browser scratch area; a calculator is optional unless rounding is requested.
- Supplied teaching case: Fictional study 2 randomly assigns volunteers to practice A or B, then uses the same test. What does random assignment primarily help address? Candidate answers: (A) It eliminates every possible study limitation (B) It makes volunteers representative of every school (C) It proves all tests measure the same thing (D) Preexisting differences between treatment groups.
- Independent check inputs: Fictional study 3 randomly assigns volunteers to practice A or B, then uses the same test. What does random assignment primarily help address? Candidate answers: (A) It eliminates every possible study limitation (B) It makes volunteers representative of every school (C) It proves all tests measure the same thing (D) Preexisting differences between treatment groups.
Learn the idea
Evidence quality depends on how participants and conditions are selected. Random sampling supports population generalization, random assignment supports treatment comparisons, and uncertainty summaries do not repair biased recruitment or uncontrolled causal differences. Random assignment distributes participants among treatment groups to reduce systematic preexisting differences. Random sampling selects participants from a population; these are different procedures with different inferential purposes.
Here is the method on the supplied teaching case. Random assignment tends to balance preexisting differences across treatment groups. Random sampling addresses population representativeness; the two randomization steps answer different questions.
Volunteer recruitment may limit generalization even after treatment assignment is random. A study can randomize one stage without randomizing the other, so state exactly which step supports a proposed conclusion.
- Write or sketch the supplied teaching case, labeling its quantities and restrictions.
- Apply the method shown in the worked solution, keeping each intermediate equality or justified inference.
- Solve the independent check with its changed inputs before revealing feedback.
- Complete the three practice cases; explain which mathematical rule rules out one distractor in each case.
See the method in action
Teaching case: Fictional study 2 randomly assigns volunteers to practice A or B, then uses the same test. What does random assignment primarily help address? Candidate answers: (A) It eliminates every possible study limitation (B) It makes volunteers representative of every school (C) It proves all tests measure the same thing (D) Preexisting differences between treatment groups. Solution: Random assignment tends to balance preexisting differences across treatment groups. Random sampling addresses population representativeness; the two randomization steps answer different questions. The final answer is Preexisting differences between treatment groups. Volunteer recruitment may limit generalization even after treatment assignment is random. A study can randomize one stage without randomizing the other, so state exactly which step supports a proposed conclusion.
Check your reasoning
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Helpful tips
- Random assignment distributes participants among treatment groups to reduce systematic preexisting differences.
- Volunteer recruitment may limit generalization even after treatment assignment is random.
See the answer and explanation
Preexisting differences between treatment groups
Random assignment tends to balance preexisting differences across treatment groups. Random sampling addresses population representativeness; the two randomization steps answer different questions.
Take it further
Changed-input task: Fictional study 9 randomly assigns volunteers to practice A or B, then uses the same test. What does random assignment primarily help address? Candidate answers: (A) It eliminates every possible study limitation (B) It makes volunteers representative of every school (C) It proves all tests measure the same thing (D) Preexisting differences between treatment groups. Show the mathematical reason, not only the selected answer. Expected result: Preexisting differences between treatment groups. Random assignment tends to balance preexisting differences across treatment groups. Random sampling addresses population representativeness; the two randomization steps answer different questions.
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