To better understand how our experimental sample compared to the average property in Dar es Salaam, we matched our experimental data to a set of data comprising every parcel in TRA’s database for the city. Out of approximately 830,000 parcels in the city, nearly 30 percent are owned by a taxpayer in our experimental sample. Parcels included in our experiment were only slightly (1.4 percentage points) less likely to have been issued a bill. Of those that were issued bills, only 35 percent of properties in the experimental sample had been valued by the TRA, as opposed to 74 percent in the rest of the city (those that were not valued were charged a TSh 10,000 flat tax). Conditional on being billed and rated, the median value for properties included in the experiment was higher, approximately 31.2 million TSh (13,500 USD) versus 20 million TSh (8,600 USD) for the rest of the city. Properties that were billed and rated faced, on average, an annual tax rate of approximately 0.16% of the property’s rated value.

# **3.2 Treatments**

In collaboration with the Tanzania Revenue Authority, we randomized each property owner into one of five groups.<sup>3</sup> The treatments are summarized in Table 1. The control was not to receive any message from the TRA. Group T1 received a simple message reminding them to pay their property tax, indicating the due date (June 30th) and providing information the taxpayers could use to contact the TRA in case they had any questions. All other treatments included this reminder message.

Treatment 2 added a ‘reciprocity’ message, where taxpayers were reminded that taxes fund social services and infrastructure and finished with the TRA’s slogan “Together we build our Nation.” Treatment 3 included the simple reminder as well as a ’social pressure’ message in which taxpayers were reminded, in a negative fashion, that non-compliers were not contributing to the development of the country or their own communities. The final planned treatment was an enforcement message which included the simple statement “Pay your tax early to avoid penalties.” However, for reasons we discuss below, during the implementation of the experiment no taxpayers were sent this message.

# **3.3 Implementation and challenges**

Following the randomization, a list of taxpayers and their phone numbers (included in the original TRA dataset) were provided to the TRA. The majority of messages were sent out after June 20, fewer than ten days before the initial deadline to pay property tax. While there was overlap in the delivery of different treatments, completion of each treatment arm proceeded sequentially, with reminder messages being sent first to group T1, reciprocity messages second to group T2, followed by social pressure messages to group T3.<sup>4</sup> There were two errors in the message delivery process. First, the firm in charge of sending the messages sent Treatment 1 messages to taxpayers who had been randomly allocate to Treatment

> 3The randomization was conducted using the Stata command `randtreat` , with “misfits” (i.e., observations beyond those that are a multiple of the number of treatment groups) being dealt with using the _strata_ method, which randomly allocates misfits across all strata (Carril et al. 2017).

> 4Figure A1 in the Appendix shows the timing of message delivery. One concern is that the differential timing of message delivery affected the impacts. In the results section, we provide suggestive evidence that our pattern of results is robust to this.

5