User`s guide

E-Prime User’s Guide
Appendix B: Considerations in Research
Page A-30
experiment and to permit questions about it. Instructions, practice, and debriefing are considered
separately below.
Instructions
The purpose of the instructions, in any experiment, is to let the subject know what will be
happening and what the correct responses are. In RT research, instructions should also
emphasize that subjects are to respond as quickly as possible while still remaining accurate.
"Accurate" is typically considered 10% or fewer errors, though this would also depend on the
specific experiment.
In long experiments, it is also advisable to instruct subjects that they should take occasional
breaks. If trials are initiated by the subjects, these breaks are under the subjects' control.
Otherwise, it is a good idea to "build in" breaks by having blocks of trials that are fairly short (e.g.,
5-10 minutes). Occasional breaks avoid having the subjects just staring at the screen and
pressing keys like zombies. This means that subjects are less error-prone, and also that RT is
less subject to added variability due to eye strain, mental fatigue, and the like.
Practice
Most experiments ask people to do unfamiliar tasks, and require them to indicate their responses
by pressing keys that have no previous association with the stimulus. If asked to press the '1' key
if a ‘C’ or ‘O’ appears and the '2' key if a ‘G’ or a ‘Q’ appears, subjects must first learn to associate
1 with C and O and 2 with G and Q. At first, subjects will be very slow and error-prone in their
responses, simply because they have to carefully think about which key to press after they
identify the target letter. After a while, subjects no longer have to think about which key to press,
and their responses become faster and more accurate. For this reason, usually give some
practice on the task before actually beginning to collect data. The effect of this practice is to
reduce the variability of RT during the experiment itself. The number of practice trials can be
determined during pilot testing. It is also a good idea to stand and watch the subject during the
practice trials, to be sure they understand the task. You may sometimes need to encourage them
to slow down, if they are making many errors. Once they clearly understand the task, encourage
them to try to speed up. Presenting the mean accuracy after each trial or block of trials can be
useful.
In a short experiment, completed in a single session, one block of practice trials is usually all that
is needed. If the experiment extends over several sessions, a brief block of practice trials is
usually given at the beginning of each session and the first session is often treated as a practice.
If the type of stimulus display or responses change from block to block, it might also be necessary
to have practice before each block of trials.
Debriefing
When an experiment is over, it is usual to debrief the subject. The debriefing typically is a simple
matter of telling the subject what pattern of RT's is expected to be found and why. That is, the
debriefing is used to explain to the subject what the experiment was about. Subjects may also be
shown their individual results. A second reason for a debriefing is to get comments from the
subjects about their own experience. While such comments may not be part of the data proper,
they can sometimes reveal the use of strategies that the experimenter had not considered, or
may even point out flaws in the design. Remember that subjects have spent some of their time
during the experiment trying to figure out "what is going on." In doing so, they may notice things
about the experiment that the experimenter never noticed—including problems.