User`s guide

E-Prime User’s Guide
Appendix B: Considerations in Research
Page A-33
accuracy trade-off, because the subjects may be sacrificing (trading) lower accuracy for greater
speed. That is, they may be faster on those trials because they are pushing themselves for
speed, but ignoring the higher error rate that often goes with that effort. Consider the comparison
of RT's in the letter-identification task.
Suppose that no differences in RT were found with increased distance from foveal vision, in
contrast to the expected finding of an increase in RT to identify letters seen less clearly. If the
error rates were seen to be increasing with incremental difference, this would suggest that
subjects were trading accuracy for speed—in order to maintain the same speed of response
under more difficult conditions, the subjects were permitting the error rates to climb.
Fortunately, in most RT research a speed-accuracy trade-off does not occur. In fact, most of the
time the fastest conditions will have the lowest error rates, while the longest RT's will come in
conditions with the highest error rates. In this case, difficult stimuli lead to both slow and sloppy
responses. In any case, it is a wise practice to examine error rates for evidence of a speed-
accuracy trade-off. To avoid this problem, instructions to the subjects usually stress that they
must be as fast as they can in each condition but without sacrificing accuracy. That is, the error
rates should be uniformly low for all conditions.
Stimulus-response compatibility
In most RT research, the connection between the stimulus and the response is arbitrary.
Subjects may be instructed to press '<' for an S and '>' for an H, or '>' for an S and '<' for an H.
But occasionally the mapping is not arbitrary. Consider the same experiment, but using L and R
as stimuli, instead of S and H. If subjects had to press '<' for an R and '>' for an L, for example,
they might be both slower and more error-prone than otherwise, because of the association of L
with "left" and R with "right." Making a "left" response to an R might well produce some response
competition, resulting in a slowing of RT. Basically, any time a stimulus implies a certain direction
of response (such as L and R implying left and right responses), there are potential problems of
S-R compatibility.
Probability of a stimulus
In most experiments with RT as a dependent variable, each type of stimulus is presented equally
often. In this way, subjects are discouraged from guessing, since each stimulus is equally likely
on each trial. Sometimes, however, one stimulus may be presented more often than another and
can have major effects on RT (and error rate). In general, the most common stimulus is
responded to more quickly and more accurately. Why is this so? Suppose that in the experiment
on recognizing S and H the subjects were presented an H 80% of the time, and an S 20%.
Subjects would quickly realize this, and would expect an H most of the time. On any trial, if the
target is an H, there is likely to be a faster response. But if the target is an S, the subjects must
overcome their expectancy, and preparation for an H. The result is a slower response, and a
higher probability of error.
Because of these considerations, it is best to always have the different trial types equally likely
whenever randomization is used. Unequal stimulus probabilities are best avoided, unless they
form part of the research itself.
Number of different responses
RT increases as the number of possible responses increases. This relationship has long been
known, and was quantified in the early 1950's, when Hick and Hyman, working independently,
each noted that RT increases linearly with the logarithm (base 2) of the number of alternatives.
That means that additional alternatives will increase RT, but the effect of that increase is smaller