User`s guide

E-Prime User’s Guide
Appendix B: Considerations in Research
Page A-25
Is a keyboard the right input device?
Typically, keyboards are used for response collection, usually limiting the allowable keys to those
used for responses. However, in many situations, a keyboard may cause problems. Subjects
can easily be confused about which keys are being used. If working in a darkened room, locating
the right keys can be difficult. If subjects have to look at the keyboard to locate the keys they
need, it can be disastrous in recording reaction times. Especially with children, the temptation to
play with the keyboard may be too great. A good alternative is to use a response box with only a
limited selection of keys, such as the PST Serial Response Box available from Psychology
Software Tools. Custom response boxes can also be made, using the Custom Expansion Kit
with the PST Serial Response Box.
The Single-Trial, Reaction-Time Paradigm
An experiment using the single-trial, reaction-time paradigm consists of one or more blocks, or
sets, of trials. Each trial consists of the presentation of at least one stimulus, and the collection of
the time required for the subject to respond. The trials vary (within or between blocks), with each
trial type representing a single level of an IV (or the unique combination of levels of two or more
IV’s). The principal DV is RT, though accuracy (usually percent error or percent correct) is also
examined as a secondary DV. Both RT and accuracy are recorded as the DV’s for each trial, but
the analysis is usually based on the mean RT (or percent correct) for each trial type, averaged
across all correct trials.
The concern for single-trial, reaction time experiments is how various independent variables
affect RT—that is, how RT is changed when we deliberately manipulate the stimuli in some way.
Inferences about cognition and perception are then made, based on the pattern of RT changes
with changes in the independent variable(s). However, RT is also affected by many variables that
are not of direct interest. These potentially confounding variables must be controlled in some way
so that they do not influence the outcome.
Following a definition of RT, we present a discussion of the events that occur in a typical RT
experiment. Then we discuss a number of confounds that can affect RT, and which must be
considered in designing experiments employing RT as a dependent variable.
RT defined. For most research in psychology, RT is defined as the time from the onset of a
stimulus to the time the subject responds. For computerized experiments, this is usually the time
from stimulus onset until a key is pressed indicating a response.
It is important to note that RT can vary, depending on the particular response required. Suppose
that there are two versions of an experiment, differing only in how the subjects respond to
indicate which of the two stimulus types has been seen. In one version, they must press the '1'
and '2' keys on the computer keyboard to indicate which type of stimulus appeared. In the other
version, they must press a lever to the left or right to indicate the stimulus. Overall RT might well
be longer in the case of the lever-press, because the mechanical resistance is higher, or because
the distance to be moved is farther, or because different muscles are employed in the two types
of responses. In this case, caution is required in comparing the results of the two experiments.
Differences in the obtained RT's might be due solely to mechanical factors, and not reflect any
differences of interest. Care is needed, then, in comparing the outcomes of experiments using
different responses. Whether a relatively fast key-press or a relatively slow lever-press was used
will affect overall RT, but in either case, the difference in time to respond to the two types of
stimuli may be about the same. In comparing experiments, then, the crucial issue is whether the
same pattern of differences in RT's is seen, rather than whether overall RT differed.