Sick and Tired: Understanding and Managing Sleep Difficulties in ...

eceived a PSQI global score ≥6 compared to 38 (45.78%) healthy controls. However,
using the more conservative cut-off criteria of ≥10, as used by Osorio et al (2006), 70
(84.34%) participants with FMS and 11 (13.25%) controls were identified as meeting
the threshold for sleep disturbance (Osorio et al., 2006). Using a 2x2 contingency table,
to explore the number of cases and control participants with clinically significant poor
sleep quality (as defined by the more stringent PSQI global score of ≥10) (Osorio et al.,
2006), an odds ratio was calculated (dividing the odds in the cases by the odds in the
controls). It was found that people with FMS were 32 times more likely to have a
clinically significant sleep difficulty than healthy controls (95% CI = 14.80 - 83.94). As
shown in Table 6, participants with FMS reported significantly poorer sleep quality than
controls across all components of sleep quality.
The effect size for the differences in the key variables of interest in the study between
the two groups; dysfunctional beliefs about sleep and perceived stress, were calculated
using the equation proposed by Rosenthal (1991).
r = z
√N
With z representing the z score from the Mann Whitney U test and N representing the
number of participants (N=166). Based on a z score of z=-4.86, there was a small effect
size for dysfunctional beliefs about sleep, r = -0.38. With a z score of z=-7.07, there was
a medium effect size for perceived stress, r = -0.55. The effect size for sleep quality
between the two groups was much larger (based on a z score of z=-9.88) at r = -0.77.
A post hoc power analysis was calculated using G*power 3.1.0 to check the power of
the study to detect true differences between the two groups at the more stringent level of
significance (p