The Effect of Peer Comparisons on the Behavior of Facilities Regulated Under the Clean Water Act: A replication

Last registered on November 19, 2025
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Pre-Trial

Trial Information

General Information

Title
The Effect of Peer Comparisons on the Behavior of Facilities Regulated Under the Clean Water Act: A replication
RCT ID
AEARCTR-0017140
Initial registration date
November 16, 2025

Initial registration date is when the trial was registered.

It corresponds to when the registration was submitted to the Registry to be reviewed for publication.

First published
November 19, 2025, 2:13 PM EST

First published corresponds to when the trial was first made public on the Registry after being reviewed.

Locations

Country
United States of America
Region

Primary Investigator

Name
Paul Ferraro
Affiliation
Johns Hopkins University
Contact Primary Investigator

Other Primary Investigator(s)

PI Name
Dietrich Earnhart
PI Affiliation
University of Kansas
Contact Investigator

Additional Trial Information

Status
On going
Start date
2024-03-04
End date
2026-12-31
Keywords
Behavior, Environment & Energy
Additional Keywords
social comparisons, social norms, water pollution
JEL code(s)
Q5, D9
Secondary IDs
Prior work
This trial does not extend or rely on any prior RCTs.
Abstract
Using estimation procedures from a pre-analysis plan and data from a randomized controlled trial with 328 municipal wastewater treatment facilities in Kansas (https://osf.io/6u7t8/), Earnhart and Ferraro (2021) estimated the average effect of a peer (social) comparison letter on a facility’s reported biological oxygen demand effluent. The peer comparison letter contrasted how the facility’s discharge-to-limit ratio under the U.S. Clean Water Act compared to the ratios of other facilities in the state. In the 18-month post-intervention period, the letter recipients reported discharge-to-limit ratios that were 8% lower, on average, than non-recipients (95% CI [-15%, -1%]). For the subgroup of monthly reporters, the letter recipients reported discharge-to-limit ratios that were 11% lower, on average, than non-recipients (95% CI [-22%, -1%]). In a new trial, The Nature Conservancy aims to replicate the Earnhart-Ferraro trial using a larger, more heterogeneous sample of monthly reporting facilities from multiple states permitted under the National Pollutant Discharge Elimination System.
External Link(s)
Pre-analysis Plan

Registration Citation

Citation
Earnhart, Dietrich and Paul Ferraro. 2025. "The Effect of Peer Comparisons on the Behavior of Facilities Regulated Under the Clean Water Act: A replication." AEA RCT Registry. November 19. https://doi.org/10.1257/rct.17140-1.0
Sponsors & Partners

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Experimental Details

Interventions

Intervention(s)
The intervention is a peer (social) comparison letter that contrasts how a polluting facility’s discharge-to-limit ratio for biological oxygen demand effluent levels under the U.S. Clean Water Act in the prior fiscal year compares to the ratios of other facilities in its state.
Intervention Start Date
2024-06-30
Intervention End Date
2025-12-31

Primary Outcomes

Primary Outcomes (end points)
The outcome variable (dependent variable) is the natural logarithm of the ith facility’s biological oxygen demand (BOD) discharge-to-limit ratio in month t. The discharge-to-limit ratio is the ratio of the facility’s reported BOD discharge in its Discharge Monitoring Report to the facility’s permitted BOD limit. See the trial's Pre-Analysis Plan for more details.
Primary Outcomes (explanation)

Secondary Outcomes

Secondary Outcomes (end points)
Secondary Outcomes (explanation)

Experimental Design

Experimental Design
National Pollutant Discharge Elimination System (NPDES) facility contact names were blocked-randomized to either a treatment (intervention) and or a control (no intervention) group in approximately equal proportions.
Experimental Design Details
Not available
Randomization Method
Block randomization on computer using Stata command - block_ra - . The blocking covariates, measured at the facility level, were the state, the sectoral subgroup, and the quartile of average FY23 BOD discharge-to-limit ratio. For NPDES contacts associated with more than one facility: the average BOD values of the facilities associated with the contact was used to obtain their quartile for blocking, the modal sectoral grouping of the facilities associated with the contact was used to obtain their sectoral group for blocking, and the modal state of the facilities associated with the contact was used to obtain their state for blocking.
Randomization Unit
NPDES facility contact name
Was the treatment clustered?
Yes

Experiment Characteristics

Sample size: planned number of clusters
4,602 facility contact names
Sample size: planned number of observations
5,076 facilities (most contact names are associated with only one facility)
Sample size (or number of clusters) by treatment arms
2,503 facilities in the treatment group, 2,573 facilities in the control group
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)
See "Section VIII Power Simulations" in trial's Pre-analysis Plan (https://osf.io/tprk5/overview).
Supporting Documents and Materials
IRB

Institutional Review Boards (IRBs)

IRB Name
Homewood Institutional Review Board
IRB Approval Date
2024-04-03
IRB Approval Number
N/A
Analysis Plan

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