Zambia VAT incentive experiments: version 2

Last registered on August 03, 2023


Trial Information

General Information

Zambia VAT incentive experiments: version 2
Initial registration date
February 27, 2023

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
March 10, 2023, 2:56 PM EST

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

Last updated
August 03, 2023, 10:56 AM EDT

Last updated is the most recent time when changes to the trial's registration were published.



Primary Investigator

Cornell University

Other Primary Investigator(s)

PI Affiliation
International Growth Centre
PI Affiliation
Zambia Revenue Authority
PI Affiliation
Cornell University

Additional Trial Information

In development
Start date
End date
Secondary IDs
Prior work
This trial does not extend or rely on any prior RCTs.
This project is on the topic of tax leakage and policy interventions to increase VAT compliance in Zambia. We will test the impact of a probabilistic incentive (similar to a lottery ticket) on the extent to which small retailers in Lusaka request VAT invoices when making purchases from their suppliers. We will examine heterogeneous impacts by some key firm characteristics measured at baseline.

In a second experiment launched concurrently with the end of the first, we will test whether a tax morale message has an impact on the value of VAT invoices retained by firms after the removal of financial incentives. This is a study of habit formation and the possibility of transitioning from extrinsic to intrinsic motivation to assist with tax compliance.
External Link(s)

Registration Citation

Dillon, Brian et al. 2023. "Zambia VAT incentive experiments: version 2." AEA RCT Registry. August 03.
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Experimental Details


All study firms are asked to request and retain receipts and invoices when making purchases from their suppliers. At each follow-up visit, treated firms are provided with the opportunity to play a lottery-style game to win a cash prize. The probability of winning cash is increasing in the total value of VAT invoices retained.

We launched a second intervention in July-August 2023, as the lottery experiment wrapped up. We asked all firms to continue retaining receipts and invoices for the coming months, but removed all financial incentives. We now randomized firms into two new groups, treatment and control, for a new experiment we call the messaging experiment. The treatment group received a message intended to prime tax morale and intrinsic motivation to support tax compliance, the control group received a placebo message.
Intervention Start Date
Intervention End Date

Primary Outcomes

Primary Outcomes (end points)
1. Total value of VAT invoices received when making purchases from suppliers or capital improvements to the business
2. Share of stocking or capital improvement transactions that lead to the receipt of a valid VAT invoice (as a share of all stocking/capital improvement purchases)
3. VAT-related shopping behaviors, including switching to a VAT-registered supplier, reallocating business to an existing VAT-registered supplier, and inquiring after the VAT registration status of an existing or potential supplier
4. The share of stocking/capital purchases from regular suppliers, where regular suppliers are those from whom the firm has made at least two recent purchases, and from which it plans to purchase again
5. The strength of relational contracts with suppliers

Update, July 2023: The primary outcomes for the messaging experiment are 1 and 2 above; the secondary outcomes for the messaging experiment are 3,4, and 5.
Primary Outcomes (explanation)
We measure the strength of relational contracts with suppliers using an index based on whether the firms' regular suppliers (1) provide goods on credit, (2) offer price discounts, (3) give favorable delivery terms, (4) provide extra products as a gift for regular business, and (5) provide other goods or services not formalized in a contract. We also investigate these outcomes separately.

Secondary Outcomes

Secondary Outcomes (end points)
Secondary Outcomes (explanation)

Experimental Design

Experimental Design
We randomize 1083 small retail firms in Lusaka, which we enrolled in a baseline survey, into three treatment arms:

Control (407 firms)
Low treatment (338 firms)
High treatment (338 firms)

Treated firms receive either a low or a high financial incentive to request and retain VAT invoices from their suppliers. Treatment assignment is at the individual firm level, stratified on key characteristics that we identified at baseline.

Updatee July 2023:

At the conclusion of the above experiment, we launched a second RCT involving a tax morale message. 816 firms were still participating as of July 2023. We randomly assigned 50% to messaging treatment and 50% to messaging control (408 each), at the firm level.
Experimental Design Details
Not available
Randomization Method
Randomization Unit
Individual firm.
Was the treatment clustered?

Experiment Characteristics

Sample size: planned number of clusters
Treatment assignment is not clustered.
Sample size: planned number of observations
1083 firms, for a minimum of 4 follow-up surveys (after the baseline survey, and a brief survey conducted during intervention delivery). Update July 2023: 816 firms, for 3-4 follow-up surveys in the messaging experiment.
Sample size (or number of clusters) by treatment arms
Not a clustered RCT.
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)

Institutional Review Boards (IRBs)

IRB Name
Cornell University
IRB Approval Date
IRB Approval Number