User`s guide

E-Prime User’s Guide
Appendix B: Considerations in Research
Page A-28
and thus not be looking at the fixation when the trials begin. This can be prevented by stressing
in the instructions that subjects should be looking directly at the fixation when they start each trial,
and also by randomly presenting the stimuli to left or right of fixation. If subjects adopt a guessing
strategy, this will lead to them missing many stimuli completely, and the high error rates will
clearly show that there is a problem.
Because of the brief displays and the need to guarantee that the subject is looking at the fixation,
subject-initiated trials should be used, with a fixed foreperiod. RT’s for this task should be fairly
fast, so it would probably be appropriate to limit the allowed response time to 2 seconds or less.
Accuracy feedback would be used, with RT reported on correct trials only.
What happens within a block of trials?
The entire series of trials making up an experiment is usually divided into blocks of trials. The
division may simply reflect time constraints. In a long experiment, it is best to ensure that
subjects take occasional pauses, so it may be best to break the entire series into shorter blocks,
with rest pauses between them. More importantly, the division of the experiment into blocks may
be an integral part of the experiment itself. The rest of this section treats that situation.
Blocked versus random presentation
Suppose there are two or more different sorts of trials being presented in an experiment (two or
more independent variables, with two or more levels of each). A question to consider is whether
these different sorts of trials should be presented together in each block with the various types
alternating in random order, or whether the series of trials should be blocked, with all trials of one
type presented, followed by all trials of the other.
Compare two versions of the letter-identification experiment. One is the experiment described
above, except that subjects must respond by pressing one of four keys to indicate either which of
four letters is present (four-choice RT). The other is the same, except that only two letters are
used (two-choice RT). The two experiments differ only in whether the two types of trials (two-
and four-choice) occur randomly (within a single block), or are blocked, with all of the two-choice
trials occurring together, and all of the four-choice trials occurring together. In order to directly
compare the four-choice RT to the two-choice RT, the two types of trials (two- and four-choice)
could occur either randomly (within a single block), or blocked, with all of the two-choice trials
occurring together, and all of the four-choice trials occurring together.
In general, we expect that RT will increase with the number of choices (Wickens, 1992). If
subjects completed one block of two-choice and one block of four-choice, that would probably be
the outcome. But with random presentation, that may not be so. Why not? In this experiment, it
is likely that random presentation would lead the subjects to ignore whether the trial is two- or
four-choice. That is, the subjects seeing the stimuli in random order may not bother to pay
attention to whether the trial involves two choices or four, but rather treat all of the trials as if they
involved four possible choices. That would increase the mean RT for two-choice trials, while
having no effect on four-choice trials. That is, the results of the experiment depend (in part) on
the choice of blocked versus random presentation of the stimulus types.
In general, then, the choice of random or blocked presentation must depend on whether subjects
given random ordering of trials will adopt different strategies than those given blocked order. In
the case of the experiment above, subjects in the random-order experiment might adopt the
strategy of ignoring whether there were two choices or four, and treat the two-choice trials as if
they were four-choice trials. Thus, the blocked version gives us the better estimate of the actual
time required for choosing between two and four response choices.