Statistical analyses
The primary outcome was recovery into the normal range of CP and
improvement of CP and DHI score at post-intervention. The standardized
mean effect was calculated using Hedges’s g value to represent the
effect sizes according to the Cochrane Handbook for Systematic Reviews
of Intervention.8 Hedges’ g value is a standardized
mean difference that was calculated to standardize the values measured
by various measurement tools into a single unit. It is also a method to
compensate for the shortcomings of the existing Cohen’s d value. In this
study, the sample size was not large. Therefore, the Hedges’ g value was
calculated. The Hedges’ g values were 0.2 as small, 0.5 as medium, and
0.8 as large. When calculating the total effect size, both the DHI score
(with the negative effect size direction) and the other variables (with
static effect direction) were included. This combination resulted in a
mutually offset effect. Therefore, DHI score was converted to the
reverse direction and analyzed.
The estimates were pooled using a fixed-effects model, which assumes
that the effect sizes of the populations are the same, and that the
differences in effect size are attributable to sampling
error.11 The homogeneity of the studies was identified
through Higgin’s I statistics and forest plots. The heterogeneity of the
studies was tested using the forest plot and Higgins
I2 statistic. Significant heterogeneity was defined as
an I2 value of 25%, 50% and 75% as low, moderate,
and high heterogeneity, respectively.12 When needed,
subgroup and sensitivity analyses were conducted. Funnel plots and
Egger’s linear regression values for each outcome were prepared and
evaluated to assess potential publication bias. All analyses were
performed using Cochrane Collaboration’s software (RevMan version 5.3.3;
Nordic Cochrane Centre, Copenhagen, Denmark) and Comprehensive
Meta-Analysis software version 3 (Borenstein M, Hedges L, Higgins J and
Rothstein H; Biostat, Englewood, NJ, USA).13
Some parts violate the independence assumption when calculating the
total effect size. However, since the number of documents included in
the analysis was limited, there was minimal information loss by using
the effect size as an analysis unit.