name of the health clinic she was assigned to attend, conditionality (Clinic condition or Vaccine condition), and the amount of cash compensation to be provided (5, 300, or 800 naira) if she satisfied the assigned conditionality.

37Upon arriving at the assigned health clinic, all women were asked to form a line to wait to be served regardless which intervention they had received. Since there is no significant difference in waiting time according to treatment status, waiting time should not cause any difference in vaccine take-ups across treatment groups.

> 38The date and time of the attendee’s visit, the means of transport from her house to the clinic, transportation costs paid, and perceptions about tetanus toxoid vaccination were also recorded. Although receiving multiple doses of tetanus toxoid vaccine is crucial for its efficacy, our experiment focused on a single-dose take-up to identify psychic costs of vaccination without introducing complexities in repeated take-up decisions. Covering multiple doses, which takes time, was also practically infeasible. The process of receiving a vaccine did not waste any time, because the interviewer was filling out the administrative information, such as the date of the interview, in the survey form, and copying the unique ID from the voucher to the survey form while the health staff were giving the attendee a vaccination at the same time, in the same place. The study made the process at the health clinic very smooth, contrary to the usual situation, for administrative purposes, and for minimizing the administrative difference between Clinic condition and Vaccine condition at the clinic. As a result, women under Clinic condition might have accepted the vaccine more than in the normal setting. We employed this design to pursue the internal validity, but at the cost of external validity.

> 39All of the women who visited the health clinics showed up at the ones they had been assigned to attend. If some women who attended the clinic failed to be interviewed for operational reasons, clinic attendance would be underestimated. We believe that such attrition is extremely rare.

40Transportation costs are self-reported costs to visit the assigned health clinic using the mode of transport the respondent would typically use. Opportunity costs are calculated as the amount of money the respondent would have earned if she had not attended the clinic, based on the daily income of the household to which she belongs, her contribution to the household income, and the time it took for her to visit the clinic.

> 41While the Nigerian DHS sampled women aged 15 to 49, we restricted the DHS sample to women aged 15 to 35 to compare with our sample. In the DHS sample, over half of women are Muslim (57.3 percent), about half (49.6 percent) have never received any education, 14 percent are pregnant, and 62 percent engage in paid work. Distinct from our sample, only a very small proportion of women in the DHS sample are single (2 percent), and most of them (96.3 percent) have at least one child. In the DHS sample, 31.8 percent of women have received a tetanus vaccination. The means of most variables in the DHS sample are not statistically different from those in our sample (results not shown).

> 42The baseline survey also asked questions regarding beliefs about vaccination in general. In the whole sample,

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