For my fMRI experiment, I need to compute the exact number of seconds (expressed in TRs) that a run of my experiment takes. I define a “run” to be a chunk of 16 trials, and there are 4 such runs in the experiment, as per the conditions file.
I computed the run duration as the number of trials times the average duration of each trial (itself a sum between a jittered-duration fixation cross, stimulus presentation, and response window), and I also looked in the log file (event timings) of one test run, to see how long the run duration actually was, between the onset of the first trial and the offset of the last. To my surprise, between the computed and the measured total run duration, the amount of time differs by around 14 seconds (270s vs 284s).
I’ve done several sanity checks but cannot figure out whence this difference arises. Could this be likely due to some aspect of the Builder that I am overlooking, or have I necessarily made a mistake somewhere in the calculation and need to look harder?
OS (e.g. Win10): Win10
PsychoPy version (e.g. 1.84.x): 3.1.5