Table 3.

Control variables across Birch engagement clusters for the pre-COVID cohort.

Variable	C0	C1	C2	C3
Pre-COVID Birch clusters
Summary statistics
Signups	2,716	1,345	798	189
Proportion of all pre-COVID signups	53.80%	26.64%	15.81%	3.74%
Signups per day	10.61	5.25	3.12	0.74
Control variables not used for clustering
Gender (p = 0.684)
Female	2091 (76.99% [75.37, 78.53])	1,045 (77.7% [75.39, 79.84])	612 (76.69% [73.63, 79.49])	481 (80.84% [77.48, 83.8])
Male	535 (19.7% [18.25, 21.24])	245 (18.22% [16.24, 20.37])	158 (19.8% [17.18, 22.71])	91 (15.29% [12.63, 18.41])
Non-binary	90 (3.31% [2.7, 4.06])	55 (4.09% [3.16, 5.28])	28 (3.51% [2.44, 5.02])	23 (3.87% [2.59, 5.73])
Age group (p = 0.052)
14–17	2,520 (92.78% [91.75, 93.7])	1,233 (91.67% [90.07, 93.03])	757 (94.86% [93.1, 96.19])	173 (91.53% [86.69, 94.72])
18–25	196 (7.22% [6.3, 8.25])	112 (8.33% [6.97, 9.93])	41 (5.14% [3.81, 6.9])	16 (8.47% [5.28, 13.31])
Ethnicity group (p = 0.01)
Asian	183 (6.74% [5.85, 7.74])	101 (7.51% [6.22, 9.04])	32 (4.01% [2.85, 5.61])	15 (7.94% [4.87, 12.68])
Black	75 (2.76% [2.21, 3.45])	37 (2.75% [2.0, 3.77])	26 (3.26% [2.23, 4.73])	3 (1.59% [0.54, 4.56])
Mixed	138 (5.08% [4.32, 5.97])	54 (4.01% [3.09, 5.2])	34 (4.26% [3.06, 5.89])	13 (6.88% [4.06, 11.41])
Other	36 (1.33% [0.96, 1.83])	29 (2.16% [1.51, 3.08])	10 (1.25% [0.68, 2.29])	3 (1.59% [0.54, 4.56])
White	2,284 (84.09% [82.67, 85.42])	1,124 (83.57% [81.49, 85.45])	696 (87.22% [84.72, 89.36])	155 (82.01% [75.91, 86.83])
Messages received
Message received	447 (16.46% [15.11, 17.9])	344 (25.58% [23.32, 27.98])	147 (18.42% [15.88, 21.26])	88 (46.56% [39.59, 53.67])

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.