Comparing Treatments Using Randomized Experiments

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Questions 1 - 10
1

A library tested two reminder methods for returning books on time. They randomly assigned 50 patrons to Treatment A (text reminder) and 50 patrons to Treatment B (email reminder). The outcome was whether the book was returned on time. Treatment A had 41 on-time returns ($0.82$) and Treatment B had 33 on-time returns ($0.66$). The observed difference in proportions was $0.82-0.66=0.16$ (A − B). A randomization test was performed by shuffling the on-time/late outcomes among all 100 patrons 4000 times under “no treatment effect.” In the 4000 shuffles, 12 produced a difference in proportions of at least $+0.16$ (A − B).

Which conclusion is most reasonable about the treatment effect (A − B)?

Because only 12 out of 4000 shuffles were at least +0.16, the observed difference would be unusual under no effect; there is evidence text reminders increase the on-time return proportion compared with email reminders.

Because 12 out of 4000 shuffles were at least +0.16, the observed difference is common under no effect; there is not enough evidence of a difference.

The difference must be due to patrons in the text group being more responsible, since confounding always explains group differences.

Text reminders are guaranteed to make exactly 16% more patrons return books on time in any library.

Explanation

This question covers comparing reminder methods in a randomized experiment and simulation to see if texts improve on-time book returns over emails. Random assignment makes groups similar, allowing attribution of differences to treatments for cause-and-effect inferences. The observed difference is in return proportions, 0.16 higher for Treatment A than B. The 'no effect' simulation shuffles outcomes across patrons multiple times, generating a distribution of possible differences by chance alone under no effect. Only 12 out of 4000 shuffles were at least +0.16, meaning the observed is unusual and provides evidence for Treatment A's effectiveness. Remember, random assignment supports causality in the study unlike random sampling for generalization, and rarity offers evidence without absolute proof. To use this approach, evaluate the count of simulations as or more extreme than observed.

2

In a randomized experiment at a community center, 80 participants were randomly assigned using a random number generator to either Treatment A (a new puzzle game) or Treatment B (a classic puzzle game), 40 in each group. After 10 minutes, success was defined as “solved the puzzle.” In Treatment A, 26 of 40 succeeded ($0.65$). In Treatment B, 18 of 40 succeeded ($0.45$). The observed difference in proportions was $0.65-0.45=0.20$ (A − B). A randomization test shuffled the success/failure outcomes among the 80 participants 5000 times under “no treatment effect.” In the 5000 shuffles, 41 produced a difference in proportions of at least $+0.20$ (A − B).

Do the results provide evidence that Treatment A is more effective than Treatment B?

Yes. Since 0.65 is 20% larger than 0.45, Treatment A is guaranteed to be better for all participants.

No. The difference in proportions must be due to confounders because the two groups might have different skill levels.

No. Because 41 out of 5000 is greater than 0, the observed difference is expected under no effect, so there is no evidence of a treatment effect.

Yes. Only 41 out of 5000 shuffles were at least +0.20, so the observed difference would be unlikely under no effect; there is evidence Treatment A increases the success proportion compared with Treatment B.

Explanation

This question examines comparing puzzle games in a randomized experiment with simulation to see if a new game increases success rates. By randomly assigning participants, we minimize biases and can reasonably conclude that differences stem from the treatments, facilitating cause-and-effect statements. The observed difference is in success proportions, calculated as 0.20 higher for Treatment A than B. The 'no effect' simulation shuffles success outcomes across all participants multiple times, building a distribution of differences that might arise purely by chance if treatments had no effect. With only 41 out of 5000 shuffles at least +0.20, the observed difference is unlikely under no effect, providing evidence that Treatment A is more effective. It's a misconception to equate random assignment with random sampling; the former supports causality in the study, and while rarity suggests evidence, it doesn't guarantee proof for everyone. In other scenarios, examine the frequency of simulated differences equaling or exceeding the observed to assess rarity.

3

In a randomized experiment, 72 volunteers were randomly assigned to two different puzzle-solving strategies: Treatment A (work backward) and Treatment B (trial-and-error), 36 per group. The outcome was the number of hints used (lower is better). Treatment A had a mean of 2.1 hints and Treatment B had a mean of 2.9 hints, so the observed difference in means was $2.1-2.9=-0.8$ (A − B). A randomization test shuffled the treatment labels 2500 times under “no treatment effect.” In the 2500 shuffles, 5 produced a difference in means of at most $-0.8$ (A − B).

Which conclusion is most reasonable about the treatment effect (A − B), where more negative values indicate fewer hints for Treatment A?

Because only 5 out of 2500 shuffles were at most −0.8, the observed difference would be unusual under no effect; there is evidence Treatment A reduces the mean number of hints used compared with Treatment B.

Because 5 out of 2500 shuffles were at most −0.8, the observed difference is common under no effect; there is not enough evidence of a treatment effect.

Treatment A is proven to reduce hints for every person because the mean difference is −0.8.

The observed difference must be due to different volunteer backgrounds, so random assignment cannot support a causal conclusion.

Explanation

This question examines comparing puzzle strategies in a randomized experiment and simulation to check if working backward reduces hints needed. Random assignment minimizes differences between groups, supporting cause-and-effect conclusions from treatments. The observed difference is in mean hints, -0.8 for Treatment A minus B, showing fewer for A. The 'no effect' simulation shuffles labels on fixed hint counts many times, building a distribution of chance differences assuming no treatment effect. Only 5 out of 2500 shuffles were at most -0.8, indicating the observed is unusual and evidencing Treatment A's benefit. A misconception is equating random assignment to random sampling; it facilitates causality in this context, with rarity providing evidence, not certainty. For transfer, focus on how often simulations are as extreme or more than observed.

4

In a randomized experiment, 90 participants were randomly assigned (45 each) to two different instruction videos: Treatment A (interactive) and Treatment B (standard). After viewing, each participant answered 10 comprehension questions. Treatment A had a mean score of 7.1 and Treatment B had a mean score of 6.2, so the observed difference in means was $7.1-6.2=0.9$ (A − B). A randomization test shuffled the A/B labels 1000 times under “no treatment effect.” In the 1000 shuffles, 8 produced a difference in means of at least $+0.9$ (A − B).

Based on the randomization test, is the observed difference surprising under no effect?

Yes. Any observed difference must be caused by the interactive video because random assignment eliminates all variability.

Yes. Since only 8 out of 1000 shuffles were at least +0.9, the observed difference would be unusual under no effect; there is evidence the interactive video increases the mean score.

No. The randomization test is invalid because it uses shuffling instead of collecting new samples.

No. Since 8 out of 1000 shuffles were at least +0.9, the observed difference is not surprising under no effect, so there is no evidence of an effect.

Explanation

This question deals with comparing instructional videos in a randomized experiment and simulation to check if an interactive one boosts comprehension scores. Random assignment equalizes groups on other factors, permitting cause-and-effect conclusions from treatment differences. The observed difference is in mean scores, 0.9 points higher for Treatment A than B. The 'no effect' simulation shuffles labels on fixed scores many times, forming a distribution of chance-based differences assuming no treatment impact. With just 8 out of 1000 shuffles at least +0.9, the observed difference is surprising under no effect, indicating evidence for Treatment A's benefit. A key misconception is confusing random assignment with random sampling; the first aids causality in the experiment, and rarity supports but doesn't prove the effect universally. Apply this by seeing the proportion of extreme simulated differences to determine rarity.

5

A student organization compared two ways to encourage event attendance. They randomly assigned 40 members to Treatment A (personalized message) and 40 members to Treatment B (generic message). The outcome was whether the member attended the event. Treatment A had 22 attendees ($0.55$) and Treatment B had 21 attendees ($0.525$). The observed difference in proportions was $0.55-0.525=0.025$ (A − B). A randomization test shuffled the attendance outcomes among the 80 members 3000 times under “no treatment effect.” In the 3000 shuffles, 1459 produced a difference in proportions of at least $+0.025$ (A − B).

Based on the randomization test, is the observed difference surprising under no effect?

Yes. Since only 1459 out of 3000 shuffles were at least +0.025, the result is very rare under no effect and suggests a real advantage for Treatment A.

No. The study cannot be used because random assignment only matters if the members were randomly sampled from the whole campus.

No. Since 1459 out of 3000 shuffles were at least +0.025, a difference like +0.025 is fairly common under no effect; there is not strong evidence that the personalized message increases attendance.

Yes. Because Treatment A’s proportion (0.55) is larger, it guarantees the personalized message causes higher attendance.

Explanation

This question involves comparing messaging strategies in a randomized experiment with simulation to see if personalized messages boost event attendance. Random assignment ensures comparability, allowing cause-and-effect attributions to treatments. The observed difference is in attendance proportions, 0.025 higher for Treatment A than B. The 'no effect' simulation shuffles outcomes among members repeatedly, creating a distribution of differences from random chance if treatments didn't differ. Since 1459 out of 3000 shuffles were at least +0.025, such a difference is common under no effect, suggesting it's not surprising and lacks strong evidence. Note that random assignment supports causality here but differs from random sampling for broader applicability, and rarity indicates evidence without proof. To apply, count the proportion of simulations equaling or surpassing the observed extremity.

6

A coach compared two warm-up routines. Using a random number generator, 34 athletes were randomly assigned to Treatment A (dynamic warm-up) and 34 to Treatment B (light jogging). Each athlete then attempted a target drill; success was “hit the target at least 7 times out of 10.” In Treatment A, 20 of 34 succeeded ($0.588$). In Treatment B, 19 of 34 succeeded ($0.559$). The observed difference in proportions was $0.588-0.559=0.029$ (A − B). A randomization test shuffled the success/failure outcomes among athletes 2000 times under “no treatment effect.” In the 2000 shuffles, 836 produced a difference in proportions of at least $+0.029$ (A − B).

Which conclusion is most reasonable about whether Treatment A is more effective than Treatment B?

Because 836 out of 2000 shuffles were at least +0.029, a difference this large is fairly common under no effect; there is not strong evidence that Treatment A is more effective.

Because 836 out of 2000 shuffles were at least +0.029, the result is extremely rare under no effect; there is strong evidence Treatment A is more effective.

Treatment A is proven to be better because its success proportion (0.588) is higher than Treatment B’s (0.559).

No conclusion can be drawn because the athletes were not randomly assigned; the coach chose who did which warm-up.

Explanation

This question investigates comparing warm-up routines in a randomized experiment with simulation to assess effectiveness in a target drill. Random assignment helps isolate treatment effects by balancing groups, supporting cause-and-effect claims. The observed difference is in success proportions, 0.029 higher for Treatment A than B. The 'no effect' simulation shuffles outcomes among athletes repeatedly, creating a distribution of differences from random chance if treatments were identical. Since 836 out of 2000 shuffles were at least +0.029, such a difference is common under no effect, suggesting insufficient evidence for Treatment A's superiority. Misconceptions include thinking random assignment equals random sampling—it enables causality here, and rarity evidences but doesn't prove effects. For other cases, look at how frequently simulations match or exceed the observed extremity.

7

A class tested two note-taking methods. The teacher randomly assigned 28 students to Treatment A (guided notes) and 28 students to Treatment B (blank notes). After a lesson, students took the same 12-question quiz. Treatment A’s mean score was 9.8 and Treatment B’s mean score was 9.1, so the observed difference in means was $9.8-9.1=0.7$ (A − B). A randomization test shuffled the A/B labels 1500 times under “no treatment effect.” In the 1500 shuffles, 735 produced a difference in means of at least $+0.7$ (A − B).

Do the results provide evidence that Treatment A is more effective than Treatment B?

No. Since 735 out of 1500 shuffles were at least +0.7, a difference like +0.7 is fairly common under no effect; there is not strong evidence that guided notes increase the mean quiz score.

Yes. Since 735 out of 1500 shuffles were at least +0.7, the observed difference is very rare under no effect, so Treatment A is more effective.

No. The comparison is invalid because the students were not randomly sampled from all students in the district.

Yes. Because the sample size is moderate (56 students), any positive difference in means must be significant.

Explanation

This question addresses comparing note-taking methods in a randomized experiment with simulation to determine if guided notes enhance quiz scores. Through random assignment, groups are balanced, enabling cause-and-effect conclusions about treatments. The observed difference is in mean scores, 0.7 points higher for Treatment A than B. The 'no effect' simulation shuffles labels on fixed scores repeatedly, producing a distribution of differences expected by chance if treatments had no influence. With 735 out of 1500 shuffles at least +0.7, the difference is fairly common under no effect, indicating not strong evidence for Treatment A. It's important not to mix random assignment with random sampling; the former allows causal claims here, and rarity suggests evidence but isn't proof. In similar analyses, check the frequency of simulated differences as extreme as observed.

8

In a randomized experiment, 60 students were randomly assigned by shuffling identical cards (30 labeled A and 30 labeled B) to either Treatment A (a new study app) or Treatment B (a standard app). After one week, each student took the same 20-question quiz. The mean score for Treatment A was 16.4 and for Treatment B was 14.9, so the observed difference in means was $16.4-14.9=1.5$ points (A − B). To test whether this difference could be due to chance, the researcher performed a randomization test by keeping all 60 quiz scores fixed and repeatedly shuffling the A/B labels 2000 times under “no treatment effect.” In the 2000 shuffles, 18 produced a difference in means of at least $+1.5$ points (A − B).

Which conclusion is most reasonable about the treatment effect (A − B)?

The result cannot be used to compare treatments because the students were not randomly sampled from all students.

Because the students were randomly assigned, Treatment A is proven to increase quiz scores by exactly 1.5 points.

Because 18 out of 2000 is not zero, the observed difference is common under no effect, so there is not enough evidence of a treatment effect.

Because only 18 out of 2000 shuffles produced a difference at least as large as +1.5, the observed difference would be unusual under no effect, so there is evidence Treatment A increases the mean quiz score compared with Treatment B.

Explanation

This question explores comparing treatments using a randomized experiment and simulation to assess if a new study app improves quiz scores. Random assignment of students to treatments helps ensure that any observed differences are likely due to the treatments themselves, allowing for cause-and-effect conclusions by balancing out other factors across groups. The observed difference is the difference in mean quiz scores, here 1.5 points higher for Treatment A than B. The 'no effect' simulation shuffles the treatment labels many times while keeping scores fixed, creating a distribution of possible differences that could occur just by chance if the treatments had no real impact. By comparing the observed +1.5 to this distribution, we see that only 18 out of 2000 shuffles were at least as extreme, indicating the result is rare under no effect and provides evidence of a treatment effect. A common misconception is that random assignment is the same as random sampling from a population, but it actually supports causal claims within the study group, and rarity offers evidence but not absolute proof. To apply this strategy elsewhere, check how often simulated differences are as extreme or more than the observed one to gauge surprise under no effect.

9

A school randomly assigned 60 students to use either Treatment A (a spaced-practice study plan) or Treatment B (a cramming study plan) for one week before a quiz. Treatment A had $n=30$ with a mean quiz score of 82 points; Treatment B had $n=30$ with a mean quiz score of 74 points. The observed difference in means was $82-74=8$ points (A − B). To test whether this difference could be due to chance under “no treatment effect,” the teacher performed a randomization test by shuffling the treatment labels 1,000 times and recalculating (A − B) each time. In the simulation, 9 of the 1,000 shuffled differences were at least as large as $8$ points.

Which conclusion is most reasonable about the treatment effect?

Because the students were randomly assigned, Treatment A is guaranteed to be better than Treatment B by exactly 8 points.

Because 9 of 1,000 is small, the observed difference is common under no effect, so there is not enough evidence that Treatment A is more effective.

Because only 9 of 1,000 shuffled differences were $8$, the observed difference would be rare if there were no treatment effect, so the results provide evidence that Treatment A increases mean quiz score compared with Treatment B.

The randomization test is not relevant because simulations are not based on real data, so no conclusion about treatment effectiveness can be made.

Explanation

In comparing treatments using a randomized experiment and simulation, we investigate if one treatment performs better than another by assigning participants randomly to groups. Random assignment helps balance out other factors, allowing us to attribute differences in outcomes to the treatments themselves rather than chance or biases, supporting cause-and-effect conclusions. The observed difference here is the difference in mean quiz scores between Treatment A and B, which was 8 points higher for A. The 'no effect' simulation shuffles the treatment labels many times, assuming treatments make no difference, and creates a distribution of possible differences that could occur just by chance. By comparing the observed 8-point difference to this simulated distribution, we see that only 9 out of 1,000 shuffled differences were as large as or larger than 8, indicating the observed result is rare under no effect. A common misconception is that random assignment is the same as random sampling from a population, but it actually helps with causal inference within the study group, and rarity provides evidence but not absolute proof of an effect. To apply this strategy elsewhere, count how often simulated differences are at least as extreme as the observed one to assess if the result is surprising.

10

A game designer randomly assigned 90 players to try either Treatment A (a new tutorial) or Treatment B (the old tutorial), 45 players per group. The outcome was whether the player completed the first level without hints.

Results: Treatment A had 33/45 successes ($73.3%$), Treatment B had 21/45 successes ($46.7%$). Observed difference in proportions (A − B) = $0.733-0.467=0.266$.

A randomization test shuffled the success/failure outcomes across groups 5,000 times under “no treatment effect.” In the simulation, 18 of the 5,000 shuffled differences were at least as large as $0.266$.

Based on the randomization test, is the observed difference surprising under no effect?

Yes. Only 18 of 5,000 shuffled differences were $0.266$, so the observed difference would be rare under no effect, suggesting the new tutorial increases the completion rate.

Yes. Random assignment means confounders must explain the difference, so the simulation is unnecessary.

No. Since 18 of 5,000 is not zero, the observed difference is expected under no effect and does not suggest a treatment effect.

No. The randomization test should count shuffled differences with absolute value $0.266$ even though the question is about A being higher, so the given count cannot be used.

Explanation

To compare treatments, randomized experiments use simulation to test if differences are real or chance-based. Random assignment balances groups, allowing cause-and-effect conclusions by minimizing biases. The observed difference here is 0.266 higher proportion of successes for Treatment A versus B. Under 'no effect,' the simulation shuffles outcomes many times, creating a distribution of chance differences. Only 18 out of 5,000 simulations had differences as large as or larger than 0.266, marking the observed as rare without an effect. A misconception is equating random assignment with random sampling; assignment enables causality, and rarity evidences but doesn't prove the effect. Apply this by counting simulated differences at least as extreme as observed in other scenarios.

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