The temporal trend of placebo response in migraine prevention from 1990 to 2021: a systematic literature review and meta-analysis with regression

Table 4 Regression model for continuous outcomes (71 studies)

Variable	Coefficient	95% CI	P-value
Constant	-1021.7	-1783.5 to -259.9	0.009
Year (Spline Term #1) per year	0.5	0.1 to 0.9	0.009
Year (Spline Term #2) per year	-1.0	-1.7 to -0.3	0.008
Year (Spline Term #3) per year	2.2	0.2 to 4.3	0.033
% Female (per .01 increase)	0.1	-0.0 to 0.1	0.176
Mean age of patients (per year)	-0.2	-0.3 to -0.0	0.043
Route of administration
Injection (reference)	0
Intravenous	2.3	1.2 to 3.5	< 0.001
Oral	-0.3	-1.4 to 0.8	0.619
Cochrane risk of bias
Low risk (reference)	0
Some concerns	-2.0	-3.3 to -0.7	0.003
High risk	-1.0	-3.4 to 1.5	0.428

The spline terms #2 and #3 can be interpreted as changes to the overall slope (presenting as Spline Term #1) at different intervals of year. See Fig. 3 for a visual display of the relationship between Year of Publication and Placebo Response. For the variable Mean Age, the model shows that for every increase of 1 year, the Placebo Response (the difference between the placebo outcome from baseline) decreases by 0.2 units. For Route of Administration, placebos administered intravenously had a mean placebo response that was 2.3 units higher compared to the referent category of Injection route
CI Confidence Interval

ISSN: 1129-2377