Experimental Design
Sample Selection and Recruitment
The experimental sample target group consists of the universe of individuals who became eligible for naturalization between July 2021 and December 2024 and did not submit a naturalization application nor were part of the previous outreach campaign. This group was identified with the support of the City of Zurich based on the following naturalization criteria: individuals who hold a type-C residence permit, have lived in Switzerland for at least 10 years and in Zurich for at least two years, and are aged 16 or older.
The survey invite (for the baseline and endline survey) is also mailed to all newly naturalization-eligible immigrants in the City of Zurich. The invitation letters and the survey are available in six languages (English, French, German, Italian, Portuguese, and Spanish) to facilitate survey participation. Individuals receive the invite in German and a second language based on their nationality. The survey can be filled out online or in a paper-and-pencil version to facilitate survey access for participants with varying levels of digital literacy. Importantly, the survey is sent out by the research team with a time lag relative to the intervention to make it ostensibly independent from the outreach campaign and minimize experimenter demand effects. Lastly, to improve survey participation, two reminders are also sent out.
Experimental Design
The study relies on a staggered design, and the experimental timeline is divided into two phases. In a preliminary step, the City of Zurich identified the newly citizenship-eligible residents that the outreach campaign will target. During Phase 1, which corresponds to the first RCT, participants are randomly allocated to the following treatment arms:
“Complexity” treatment;
“Support” treatment;
“Requirements” treatment;
“Benefits” treatment;
Pure control group.
Each of the four information treatments corresponds to a tailored leaflet, which is sent to participants in that treatment arm. The leaflets address four perceived hurdles based on insights from a previously evaluated campaign, focus group discussions with different immigrant groups, and expert interviews. These include the perceived complexity regarding the time and paperwork needed to apply, the unawareness of available support options, the lack of knowledge about application requirements, and the perceived lack of benefits.
Participants in the four treatment groups receive either of the leaflets via mail, along with a letter from the City mayor informing them that they fulfill the eligibility criteria and encouraging them to apply. The letter addresses the perceived unwelcomeness to apply for naturalization, which can act as an additional barrier to initiating the naturalization process. The City of Zurich sent the information letters with the four randomly allocated leaflets to the recipients in the treatment groups between the end of October and the beginning of November 2024.
Participants in the pure control group do not receive any information materials during Phase 1 of the project and will form the target sample of the second RCT. In Phase 2, we will use results from Phase 1 to inform the design of two alternative treatments that explore alternative approaches to the optimal combination and targeting of information. These treatments aim to compare the effectiveness of a prescriptive and targeted approach to information provision to an untargeted information acquisition approach driven by participants.
The unit of randomization is the street-address cluster. Cluster-level randomization ensures that everyone living in the same apartment building will receive the same treatment, minimizing the scope for unintended spillovers across treatment groups or between treatment and control groups. In addition, the sample is stratified according to five regions of origin, which group together similar countries of origin. These regions could influence the primary outcome – naturalization applications – by affecting the perceived hurdles and incentives to apply for citizenship. Stratified randomization ensures a balanced distribution of regions of origin across experimental groups, reducing the potential for confounding by chance.
To complement the analysis of register data on applications for Swiss citizenship with data about recipients’ beliefs, we designed a two-wave survey. The first-wave baseline survey was disseminated to all study participants in August 2024. The second-wave endline survey will be disseminated to all study participants in August 2025, approximately 9 months after treated participants in the Phase 1 RCT received the information treatments.
Outcome Data Collection
We will collect administrative data and survey responses. The primary outcome of interest is whether a participant applies for naturalization, based on administrative records provided by the Canton of Zurich of whether and when participants apply for Swiss citizenship. To estimate the effects of Phase-1 interventions, we will analyze application data approximately six months after letters were sent out to participants in the four treatment groups (mid-April – beginning of May 2025). The results of this analysis will also inform the design of the Phase-2 interventions, which will be implemented in June 2025.
The timing of the endline survey will allow us to complement the analysis of application data with data on individuals’ beliefs 9 months after Phase-1 interventions. The survey covers questions on socio-demographics, integration, marginalization, and the expected costs and benefits of naturalization. Using items from the IPL-12 integration index (Harder et al., 2018), we will measure immigrants’ economic, linguistic, political, psychological, social, and navigational integration. These short modules (two to four questions each) are designed to capture key dimensions of integration, understood as immigrants’ capacity and knowledge to build a self-determined and successful life.