Searching for the optimal measuring frequency in longitudinal studies -- an example utilizing short message service (SMS) to collect repeated measures among patients with low back pain

Table 2 Distribution of subjects in clusters formed by the reference data (A) and the incomplete data sets (B–F)

	Reference, A, the full data set, 26 weekly measures				Kappa Agreement (Weighted Kappa)
	Cluster 1 “Fast improvers”	Cluster 2 “Normal improvers”	Cluster 3 “Slow improvers”	Cluster 4 “Indifferent”	Kappa Agreement (Weighted Kappa)
B, first 8 weeks
Cluster 1	*N =* 11	N = 12	N = 0	N = 1	0.272 (0.548)
Cluster 2	N = 5	*N =* 29	N = 8	N = 0
Cluster 3	N = 1	N = 9	*N =* 9	N = 13
Cluster 4	N = 0	N = 13	N = 6	*N =* 12
C, first 13 weeks
Cluster 1	*N =* 15	N = 3	N = 0	N = 0	0.348 (0.611)
Cluster 2	N = 0	*N =* 23	N = 2	N = 5
Cluster 3	N = 0	N = 33	*N =* 7	N = 0
Cluster 4	N = 2	N = 4	N = 14	*N =* 21
D, first 8 weeks + monthly thereafter
Cluster1	*N =* 14	N = 4	N = 0	N = 0	0.618 (0.720)
Cluster 2	N = 1	*N =* 52	N = 8	N = 2
Cluster 3	N = 0	N = 1	*N =* 8	N = 2
Cluster 4	N = 2	N = 6	N = 7	*N =* 22
E, every other week
Cluster 1	*N =* 16	N = 15	N = 0	N = 0	0.642
Cluster 2	N = 1	*N =* 45	N = 7	N = 3	(0.823)
Cluster 3	N = 0	N = 1	*N =* 13	N = 0
Cluster 4	N = 0	N = 2	N = 3	*N =* 23
F, first 18 weeks
Cluster 1	*N =* 15	N = 1	N = 1	N = 0	0.611 (0.708)
Cluster 2	N = 0	*N =* 47	N = 3	N = 1
Cluster 3	N = 0	N = 7	*N =* 8	N = 1
Cluster 4	N = 2	N = 8	N = 11	*N =* 24

Figures in bold show number of subjects in B–F that are classified in clusters with similar trajectories as A

ISSN: 1471-2288