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Field
Randomization Method
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Before
We employ stratified randomization to ensure balance across key factors that may influence donations and visitor engagement. The stratification variables are:
1) Day type (weekday vs. weekend),
2) Calendar week (first vs. second week of the study: Sep 28–Oct 4 vs. Oct 5–Oct 11, 2025), and
3) Number of hippos on exhibit in the hippo house (recorded daily).
Within each stratum, days are randomly assigned to treatment and control groups in approximately equal numbers (e.g., 3 vs. 4 days), using computer-generated random numbers.
We further checked for balance in expected attendance across treatment and control days using visitor counts from the same calendar period in the previous year (September 29–October 12, 2024, also spanning two weeks starting on the last Sunday of September). The average number of visitors on days assigned to the control group in the current year was 3,644.6, compared to 3,850.1 on days assigned to the treatment group. Statistical tests showed no significant differences, suggesting that treatment and control days are comparable in terms of expected attendance.
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After
We employ stratified randomization to ensure balance across key factors that may influence donations and visitor engagement. The stratification variables are:
1) Day type (weekday vs. weekend),
2) Calendar week (first vs. second week of the study: Sep 28–Oct 4 vs. Oct 5–Oct 11, 2025), and
3) Number of hippos on exhibit in the hippo house (recorded daily).
Within each stratum, days are randomly assigned to treatment and control groups in approximately equal numbers (e.g., 3 vs. 4 days), using computer-generated random numbers.
We further checked for balance in expected attendance across treatment and control days using visitor counts from the same calendar period in the previous year (September 29–October 12, 2024, also spanning two weeks starting on the last Sunday of September). Statistical tests showed no significant differences, suggesting that treatment and control days are comparable in terms of expected attendance.
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