1) We will analyze the impact of the reform's details for the three groups separately (i.e., for those with low property valuations and tax reductions, for those with mid-value valuations and no changes, and for those with high property valuations and tax increases).
There are two reasons to explore this heterogeneity. First, the most interesting group is the "mid-value" one, because it is the only group that does not have "skin in the game". That is, if there is a treatment effect in this group, it will not be explained by any change in the tax they have to pay. On the other hand, the tax reform directly affected the rate that the other two groups have to pay: low property valuations will see a reduction while high-property valuations will see increases.
Another reason to analyze the effect separately is that the sign of the effects could differ by group. In principle, the sign of the effect is ambiguous for every group. That is because, while some individuals may prefer a more progressive tax scheme (and thus be more willing to pay taxes when they are informed about the reform), the opposite could be true for individuals that prefer less progressive tax schemes. This ambiguity is true for the tree groups (low, mid, high property valuations).
However, in addition to this, there are two factors that are relevant specifically for the low and high property valuation groups. Individuals in the high property valuation group (for whom tax rate increased) will have an additional incentive to reduce their willingness to pay, while the opposite is true for individuals in the low property valuation group (for whom tax rate was reduced).
In any case, we will also present the pooled results to maximize power (average effects for (low, mid, high).
2) We will explore the effects by the level of "past compliance". We expect the effect to be maximized among those that were not "never payers" (that is, we expect a smaller effect among individuals that never paid taxes during 2022). This heterogeneity is based on the result of a previous experiment by Cruces et al (2022, available at https://ifs.org.uk/publications/design-two-stage-experiments-application-spillovers-tax-compliance) which found that result (which was also pre-registered) in a very similar setting.
3) The treatment letter and the survey will also be sent by email to a subsample of about 15.000 taxpayers with registered email addresses after the mailing campaign. The impact of the treatment will be gauged on survey outcomes (respondent's views on taxation, public goods and progressivity) but also on payment behavior for the second month after the main mailing treatment.
EXPERIMENTAL DESIGN UPDATE-MID FEBRUARY2023
Given the low take up for the survey from the link included in the paper letter, we will only use survey responses from the about 15,000 taxpayers with registered email addresses (see secondary outcomes above in the original resgitration). We sent a reinforcement of the treatment including the survey link by email to the subgroup of accounts with emails registered for communications with the Municipality regarding tax matters. We sent the message corresponding to the random group to which each account was originally assigned. The treatment message and the survey link were highly salient in the message. We sent a total of 14646 emails on February 23, and repeated the same message on March 10. This timing implies that these messages will only impact on the second payment in the year - the first payment will only be affected by the original paper letter treatment. Since a read receipt will be available for the email sample, we will be able to compute treatment on the treated estimates with instrumental variable for this sample.