reproduced in an independent sample [ 42, 43]. With 18
potential predictors, a sample of n = 180 reporting
healthcare utilization at follow-up would be sufficient
for the proposed analyses. However, this estimate is con-
servative. Other methods for determining sample size
for prediction analyses suggest an overall sample size of
N >5 0+8 * m (where m = number of independent vari-
ables) [ 44]o r N > 104 + number of independent predic-
tors [ 45, 46]. For these less conservative estimates, the
projected study sample size is sufficient for the proposed
analyses.
Results
Four hundred and forty subjects were recruited at initial
evaluation. Follow-up at 4 weeks was 75.0% ( n =3 3 0 ) , a t
6 months was 69.0% ( n = 304) and at 12 months was
65.2% (n = 287). Baseline demographics and health-related
characteristics for the full cohort, as well as those who did
and did not complete all follow-up are presented in
Tables 1, 2 and 3. Those who did not complete follow-up
were younger, more likely to be non-white, had less than a
college degree, were more likely to have had sudden
symptom onset, had higher baseline pain intensity, and
had higher baseline pain-related psychological distress
measured by the OSPRO-YF. Only those with complete
follow-up data at each time point were considered for pre-
diction analyses ( n = 246, 55.9%).
Overall, 43.1% ( n = 106/246) of those with complete
follow-up data utilized at least one healthcare service
following the physical therapy episode. Distribution of
utilization for specific services is provided in Table 4.
For multivariate analyses, all VIFs were less than 10 and
tolerance values greater than 0.1 suggesting no signifi-
cant multicollinearity among independent variables.
Full multivariate model performance
Overall performance for each full multivariate model is
listed in Table 5. Block 1 (Demographic, clinical and co-
morbidity) consistently contributed to prediction of
healthcare utilization and accounted for the greatest
amount of variation in utilization outcome for all
models. Block 4 (change scores for pain, disability, and
OSPRO-YF) provided statistically significant contribu-
tions in all models except prediction of injection. Blocks
including baseline OSPRO-YF and OSPRO-ROS, both
short and long forms, did not predict utilization out-
comes. Weighted models consistently outperformed
their complete case analysis model counterparts with
overall model pseudo-R 2 values ranging from .337 (Any
care) to .611 (Emergency room).
Table 1 Demographic information for the full cohort, and for those with complete and incomplete follow-up
Variable Label Full cohort at baseline
(n = 440)
Completed follow-up
(n = 246)
Did not complete follow-up
(n = 194)
p-value a
Demographic information
Age Mean ± SD 45.06 ± 15.82 46.59 ± 16.00 43.15 ± 15.43 0.02
Median (min, max) 45 (18 – 75) 47 (18 – 75) 42 (18 – 74)
Sex (1 missing) Male 164 (37.3%) 85 (34.6%) 79 (40.7%) 0.20
Female 275 (62.5%) 160 (65.0%) 115 (59.3%)
Race (7 missing) White 343 (78.0%) 200 (81.3%) 143 (73.7%) 0.05
Non-white 90 (20.5%) 42 (17.1%) 48 (24.7%)
Ethnicity (33 missing) Hispanic or Latino 31 (7.0%) 20 (8.1%) 11 (5.7%) 0.36
Not Hispanic or Latino 376 (85.5%) 211 (85.8%) 165 (85.1%)
Education (6 missing) Less than college graduate 161 (36.6%) 71 (28.9%) 90 (46.4%) < 0.001
College graduate or higher 273 (62.0%) 172 (69.9%) 101 (52.1%)
Income (66 missing) $35,000 or less 112 (25.5%) 62 (25.2%) 50 (25.8%) 0.30
$35,000 to $70,000 106 (24.1%) 59 (24.0%) 47 (24.2%)
Greater than 70,000 156 (35.5%) 99 (40.2%) 57 (29.4%)
Insurance (26 missing) Private 273 (62.0%) 156 (63.4%) 117 (60.3%) 0.70
Public 75 (17.0%) 46 (18.7%) 29 (14.9%)
Other 66 (15.0%) 36 (14.6%) 30 (15.5%)
Geographic region Southeast 275 (62.5%) 146 (59.3%) 129 (66.5%) 0.10
Midwest 47 (10.7%) 23 (9.3%) 24 (12.4%)
West 98 (22.3%) 65 (26.4%) 33 (17.0%)
Northeast 20 (4.5%) 12 (4.9%) 8 (4.1%)