User`s guide

E-Prime User’s Guide
Chapter 2: Using E-Studio
Page 38
2.5.2 Set the weights
To change the relative frequency of trial types, change the weight of the exemplars. In this
example, this is done by setting the values for the Weight attribute in the DesignList. Set the
weights for the exemplars in the Word condition to “1,” and the weights for the NonWord
exemplars to “2,” making the ratio of non-word to word stimuli 2 to 1.
2.5.3 Set the sampling mode and exit condition
The sampling mode allows the altering of the order in which levels or conditions are run.
Sampling modes include sequential, random (without replacement), random (with replacement),
counterbalance, offset, and permutation. The default sampling mode is sequential presentation
of items. This is useful for debugging to check each condition.
The counterbalance option picks one entry based on a selector variable such as subject number
or session number. For example, a design running six conditions with counterbalance by subject
would result in only the third condition being presented to subject number 3. This might be used
for a Latin Square design between subjects.
The Offset option enters the List with an offset based on the selector variable (e.g., subject
number), then samples the List in fixed order from that location for the specified number of
samples, starting with the row corresponding to the subject number, and wrapping around to the
beginning of the List when the end of the List is reached. For example, a design running six
blocks of trials with Offset by Subject would result in the first sampled item being determined by
the subject number. For subject number 3, the third item would be selected first, and the List
selection would continue in fixed order (i.e., the fourth item would be selected next, followed by
the fifth, etc.) until the end of the List is reached. The sampling would then wrap to continue with
the first item in the List, then the second item, and conclude (i.e., all of the items in the List have
been sampled). This might be used for a Latin Square design within subject assignment of
conditions.
Finally, using the selector variable, the Permutation option selects one combination from all of the
possible combinations of conditions. The Permutation option generates all possible combinations
of conditions. From the pool of possible combinations, one combination is chosen based on the
value of the selector variable. For example, a design running three blocks of trials (A, B, and C)
with Permutation by Subject would result in the generation of six possible combinations of
conditions (i.e., ABC, ACB, BCA, BAC, CAB, CBA). From those possible combinations, subject
number 3 would receive the third combination (i.e., BCA). Care should be taken when using the
Permutation option, since the generation of all possible conditions increases factorially. That is, a
large number of conditions will result in a tremendously large number of combinations of those
conditions (e.g., 5 conditions result in 120 combinations, 6 conditions result in 720 combinations,
etc.). The Permutation option is best used with a small number of conditions.