I am in the last phases of my master’s thesis, and came across a question regarding my data analysis results that I cannot seem to find a clear answer for.
For context, my data is from a randomised, double-blind, placebo-controlled, between-subject study, in which 50 healthy subjects were randomly assigned to receive either four 15μg microdoses of LSD or four doses of placebo over the course of two weeks (4 test days; baseline, dose 1, dose 4, follow-up). I used mixed model repeated measures ANCOVA with Drug (PLA, LSD) as between-subject factor, and Testday (dose 1, dose 4, follow-up) as within-subject factor. At recommendation of a statistics professor at my uni, I included the scores from the baseline test day as covariate for each outcome measure.
While the covariate itself was significant in all my analyses, the covariate:Testday interaction was not signficant for most of my variables. Except one – number of attention lapses in the PVT.
Edit as requested by @EdM
Definition/explanation of the task of the outcome variable:
[The Psychomotor Vigilance Task (PVT) measures sustained attention by assessing the reaction time in response to a visual stimulus (Dinges & Powell, 1985). Participants had to press a response button as quickly as possible when a visual stimulus occurred. The stimulus consisted of a counter that increased by 1 each millisecond (ms), and was presented a total of 100 times at random time intervals over a period of 10 minutes. Average reaction time in ms and the number of attention lapses were recorded as outcome measures. An attention lapse consists of failure to react or a reaction time greater than 500 ms.]
get_anova_table from the
rstatix package to run the ANCOVA analyses (based on this guide).
The relevant outcome measure for my question is lapses, i.e. the number of times a subject failed to react or had a reaction time of over 500ms. Condition refers to the group (placebo or LSD), Testday is, well, the test day (within-subject factor as described above), and PPNR is short for "participant number" which was the case identifier. Finally, lapses_t01 is a separate variable that contains lapses scores from the baseline test day; it was made specifically to serve as baseline covariate.
The ANCOVA for lapses gave the following R-output:
My question is now: What does the significant interaction effect of the covariate (lapses_t01) and time-factor/repeated measures variable (Testday) mean?
I have searched this site repeatedly already, but was unable to draw a firm conclusion from the explanations on any similar questions I could find. A few even seem to contradict each other; some say this just means that the influence of the covariate varies depending on Testday, while others say that this indicates a violation of ANCOVA assumptions.
I would be very grateful to learn how to interpret this result correctly.