Analysis type: linear mixed-effects model
DV: cortisol (4 time points per participant - morning, noon, 5pm, 9pm)
IV 1: time-since-waking (continuous, repeated measure, variably spaced/unstructured time variable)
IV 2: prior-day bedtime (9pm) cortisol levels
IV 3: interaction between IV 1 and IV 2
Results: significant interaction term
Question: is the relationship of prior-day bedtime PM cortisol with next day diurnal slope stronger than or independent of the relationship between PM cortisol the day before and PM cortisol the next day? In other words, is it just driven by the consistency of high PM cortisol, or is it more than that?
A) Is there a way to address the above question and is it relevant? Do I need to adjust for that?
B) I (1) extracted subjects’ predicted slope coefficients from the mixed model (ie., time-since-waking, on a model that only had time-since-waking as a predictor) and (2) ran a partial correlation looking at the relationship between prior-day bedtime cortisol levels and slope coefficients, controlling for the current day’s PM cortisol level (9pm sample, used in calculating the slope coefficients). The partial correlation was not significant and also in the opposite direction of the interaction.
I am not sure what to make of this or how to address the above question.