Table 1. Participants’ characteristics and recruitment behaviour as observed during RDD survey in the Netherlands.

Variables	Seeds in wave 0**	Recruitees in waves 1 to 6**	Overall (seeds + recruitees)*
Number of female participants (%)	662 (65.2%)	277 (64.0%)	939 (64.8%)
Mean age in years of participants (SD; range)	55.5 (13.0; 4–97)	49.4 (16.8; 3–82)	53.7 (14.5; 3–97)
Number of participants in age group 0–39, a1 (%)	127 (12.5%)	121 (27.9%)	248 (17.1%)
Number of participants in age group 40–59, a2 (%)	465 (45.8%)	173 (40.0%)	638 (44.1%)
Number of participants in age group 60+, a3 (%)	423 (41.7%)	139 (32.1%)	562 (38.8%)
Number of participants with an academic education, B¥	596 (58.7%)	234 (54.0%)	830 (57.3%)
Number of participants with a negative vaccine belief	482 (47.5%)	293 (67.7%)	775 (53.5%)
Mean number of invitations sent out, $\overline{x}$ (s2)	1.33 (3.24)	1.47 (3.23)	1.36 (3.21)
Mean proportion invitation was accepted, p (range)	18.9% (0%– 26.9%)	20.1% (7.7%– 31.6%)	19.2% (10.0%– 27.2%)

*A stratification by recruiter characteristics can be found in S1 Table.

**A stratification by recruiter characteristics and by seeds and recruitees can be found in S2 Table.

^¥Academic education defined as acquired a bachelor degree or higher in higher education.