Starting from a broader set of at-risk taxpayers, we performed two restrictions to obtain the final experimental sample. First, in order to avoid spillovers between treatment and control units, only taxpayers with a unique primary email address were included in the experimental sample<sup>19</sup> . Second, power calculations exercises suggested we could significantly increase minimum detectable effects (MDE) by dropping extremely large taxpayers. Figure A5 reports MDE for our four main outcomes of interest when we trim our sample at different percentiles. Considering all primary outcomes, we decided to trim the sample at the 97th percentile of declared revenue distribution, arriving at our final sample of 31,396 taxpayers<sup>20</sup> .

We implement a stratified randomization, at the taxpayer level, using 60 strata defined by whether third-party information was available or not; whether the taxpayer was a corporation; municipality of operations defined as Distrito Central (capital Tegucigalpa), San Pedro Sula (second largest city, often referred as the industrial capital) or other; and five risk levels as defined by the tax authority (2*2*3*5 = 60 strata). In each strata 49 percent of taxpayers were allocated to the control group and the remaining 51 percent in three equally sized treatment arms. Following Bruhn & McKenzie (2009) we deal with "misfits" (remaining taxpayers in each strata) by randomly assigning them to one of the four groups (control + 3 treatments) using the above weights as assignment probabilities<sup>21</sup> .

# **3.3 Benchmarking results: Expert forecast survey**

In order to benchmark the magnitude of our results, we follow DellaVigna & Pope (2018) and DellaVigna et al. (2020) and surveyed academics and policymakers on their predictions about the impacts of this experiment. We focused on collecting forecasts from three groups of subjects: academic economists (faculty, PhD students and researchers at academic institutions); public sector workers in Honduras, in particular those working at the tax authority; and policymakers or researchers in international development organizations<sup>22</sup> .

The main results of the survey are presented in Table 7. In terms of magnitudes of

> 19 Approximately 4,400 taxpayers were also deemed at-risk of non-compliance but shared a primary contact email with other units, either due to joint ownership or an accounting firm as primary contact. These taxpayers were excluded from the main sample and were assessed in a separate, smaller experiment to estimate spillover effects.

> 20Details about power calculations used in Figure A5 are discussed in Appendix C. Taxpayers with declared gross revenue above L19.4 million (approximately USD 780,000) in either FY2017 or FY2018 were excluded from the sample

> 21The randomization was implemented in Stata using the _randtreat_ command to deal with misfits.

> 22We sent the invitation to take the survey to approximately 120 academics, 100 practitioners and 50 public sector workers. We received 111 partial survey answers and dropped 35 incomplete surveys and 11 with mostly inconsistent answers. Our final sample consisted of 65 complete surveys. Details about the exclusion process, further trimming of outliers and complete survey questionnaires are provided in Appendix E.

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