Note. Colored lines indicate individual participants. Black lines indicate the predicted interaction effect based on the mixed effect model depicted in Table 2. The waitlist period occurred between sessions W1 and T1, while the heart rate variability biofeedback intervention period took place between T1 and T5. PAF – premenstrual assessment form; *** – p<.001; ns – not significant.
3.3 Depressive Symptoms
The final model predicting depressive symptoms included only TREATMENT and TIME, as well as their interaction as fixed effects (see Table 4). The model was the following:
Value ~ treatment*time + (1|vpn) + (1|item).
A post-hoc Tukey test of the interaction showed that symptom scores significantly improved in the intervention period, d = -0.25, tratio(1576) = -4.71, p < .001, but not in the wait-list period, d = 0.05, tratio(1567) = 0.73, p = .46 (see Figure 4).
Figure 4. Course of depressive symptoms