User`s guide

E-Prime User’s Guide
Appendix B: Considerations in Research
Page A-29
When blocked presentation is used, the issue of counterbalancing of treatment orders is raised.
In the blocked version of the two- versus four-response experiment (two levels of one
independent variable), half of the subjects would do the two-choice trials first, while half would do
the four-choice trials first. This counterbalancing is designed to remove (or at least balance) any
effects of carry-over from one block of trials to the next.
Certain confounding variables are usually controlled by counterbalancing. One is the mapping of
stimuli to responses. If interested in comparing the speed of reactions to the targets 'C' and ‘O’ to
-response version of the experiment above, have
half of the subjects respond by pressing the '1' key for ‘C’ and ‘O’ and the '2' key for ‘G’ and ‘Q’.
Half would respond in the opposite way, pressing the '2' key for ‘C’ and 'O.' This controls for any
possible difference in RT due to the different responses themselves, and is necessary because
some muscular actions take longer than others.
If comparing detection of letters under the conditions in which the letters either were or were not
adjusted in size, the comparison of interest is adjusted vs. constant size; therefore, since the ‘1’-
and ‘2’-response trials will be averaged together, counterbalancing may not be necessary. In
other experiments, however, it can be absolutely crucial. Consider, for example, a version of the
letter-choice experiment in which two letters are presented and the subject must indicate the
letters are the same by pressing one key or that they are different by pressing another. Since
one aspect of that experiment is to compare "same" and "different" responses, it would be
important to counterbalance the mapping of the response keys to same and different stimuli.
Otherwise, a difference between RT to "same" and "different" might be interpreted as reflecting
the difference in the stimuli, when it was really reflecting a difference in response time to press
the ‘1’ key versus the ‘2’ key. The difference in RT really is due to a lack of proper
counterbalancing. (Alternatively, a failure to counterbalance might lead to a finding of no
difference, when there really was one.)
Ordering of trials within a block
When each type of trial is presented within a single block of trials, it is almost always the practice
to randomize the order of trials. This is equivalent to writing down each trial on a card (including
multiple cards for repetitions of the same stimulus) and then shuffling the cards. There is,
however, a problem that can be caused by randomization. Suppose that there are two types of
trials. In a single block, 100 trials of each type are presented, in a random order. It is likely that
some fairly long sequences of a single trial type will occur, with a single type presented 7 or 8
times in a row. Because humans expect randomness to produce shorter sequences than it really
does, subjects tend to fall into the gambler’s fallacy. If a single trial type occurs 6 times in a row,
subjects will often either decide that the other trial type is “overdue” and expect it, or they will
decide that the type they have seen is more likely to occur and expect it again. In either case, if
the expectation is correct, the subject will probably be very fast and accurate. If the expectation
is wrong, the subject will be slow and error-prone. E-Prime permits setting a maximum on the
number of repetitions of a single trial type, as a way to avoid this problem.
What happens within the whole experiment?
An experiment is composed of one or more blocks of trials. If the experiment is particularly long,
it may be broken down into sessions of one or more blocks each. In that case, counterbalancing
of blocks across sessions may also be required. An experiment most often begins with
instructions about the nature of the experiment, and some practice trials. When the experiment is
concluded, some form of debriefing is often used to show the subject the purpose of the