Table 5.

Control variables across Birch engagement clusters for the COVID cohort.

Variable	C0	C1	C2	C3
COVID Birch clusters
Summary statistics
Signups	7,099	3,847	2,597	298
Proportion of all COVID signups	51.29%	27.79%	18.76%	2.15%
Signups per day	12.45	6.75	4.56	0.52
Control variables not used for clustering
Gender
Female	5,219 (73.52% [72.48, 74.53])	2,925 (76.03% [74.66, 77.36])	2026 (78.01% [76.38, 79.56])	236 (79.19% [74.23, 83.42])
Male	1,451 (20.44% [19.52, 21.39])	704 (18.3% [17.11, 19.55])	420 (16.17% [14.81, 17.64])	54 (18.12% [14.16, 22.89])
Non-binary	429 (6.04% [5.51, 6.62])	218 (5.67% [4.98, 6.44])	151 (5.81% [4.98, 6.78])	8 (2.68% [1.37, 5.21])
Age group (p = 0.109)
14–17	6,311 (88.9% [88.15, 89.61])	3,413 (88.72% [87.68, 89.68])	2,316 (89.18% [87.93, 90.32])	250 (83.89% [79.29, 87.63])
18–25	788 (11.1% [10.39, 11.85])	434 (11.28% [10.32, 12.32])	281 (10.82% [9.68, 12.07])	48 (16.11% [12.37, 20.71])
Ethnicity group (p = 0.016)
Asian	432 (6.09% [5.55, 6.67])	282 (7.33% [6.55, 8.2])	190 (7.32% [6.38, 8.38])	13 (4.36% [2.57, 7.32])
Black	255 (3.59% [3.18, 4.05])	125 (3.25% [2.73, 3.86])	81 (3.12% [2.52, 3.86])	10 (3.36% [1.83, 6.07])
Mixed	367 (5.17% [4.68, 5.71])	197 (5.12% [4.47, 5.86])	150 (5.78% [4.94, 6.74])	13 (4.36% [2.57, 7.32])
Other	246 (3.47% [3.06, 3.92])	125 (3.25% [2.73, 3.86])	65 (2.5% [1.97, 3.18])	10 (3.36% [1.83, 6.07])
White	5,799 (81.69% [80.77, 82.57])	3,118 (81.05% [79.78, 82.26])	2,111 (81.29% [79.74, 82.74])	252 (84.56% [80.02, 88.22])
Messages received
Message received	5,217 (73.49% [72.45, 74.5])	3,022 (78.55% [77.23, 79.82])	2,147 (82.67% [81.17, 84.08])	273 (91.61% [87.91, 94.25])

Reported values are absolute counts and percentages with 95% confidence intervals. Chi-squared tests across clusters returned p < 0.001 for all variables except otherwise specified.