Designing a Stroop Effect Experiment
Your task is to exactly replicate the following experiment about the Stroop Effect using the computer program “E-Prime”.
The number oriented Stroop Effect uses numbers instead of words or colors to measure interference in thought process. Your participants will see a string of numbers appear on the screen in different string lengths (the numbers of numbers in a row). You will use the numbers 3, 4, 5, and 6 (for congruent and incongruent conditions) and the character * (for the control condition) in one of four string lengths: 3, 4, 5, or 6 characters long. For example, if the combination “33333” appears on the screen, the correct answer is “5” because the string length is 5 characters long. If the combination “666” comes on screen, the participants should hit the key corresponding to “3” because the string length is 3 characters long. They should be instructed to try to ignore the number that appears and only concentrate on the string length, and to respond as quickly as possible without sacrificing accuracy.
The subject will see one of 16 possible # (or *)/string length on the screen written in one of four different string lengths: 3, 4, 5, or 6 characters. The total possible number of combinations is 20 (see the table provided for a list of all possible combinations). The experiment consists of three conditions to which the #/string length on the screen can belong (i.e. three levels of the independent variable level): (on pg. 2)
1) control: a * which appears on the screen will always be a string either
*** (3 stars)
**** (4 stars)
***** (5 stars)
****** (6 stars)
2) congruent: the number which appears on the screen and the string length (how many #’s in a row) will be the same, such that it will always be one of the following: (# appearing x the length of the string)
333 (3x3)
4444 (4x4)
55555 (5x5)
666666 (6x6)
3) incongruent: the number and the string length will always conflict, for example:
3333 (3x4)
444444 (4x6)
555 (5x3)
6666 (6x4)
The computer should measure the time from when the #/string length appears on the screen to when the participant hits the key (i.e. reaction time, or latency time) and accuracy of response – these are our dependent variables. Immediately after the participant hits the key which corresponds to the string length, the screen should move on to the next # (or *)/string length combination. When designating which key corresponds to which string length, use the number pad on the far right hand side of the keyboard.
There will be 2 blocks of trials. The first block will be a practice block of 12 trials (4 congruent, 4 incongruent, and 4 control) which will not be measured for response time or accuracy. The second block is a “real” experimental block. The computer will automatically start recording reaction time (RT) and accuracy with this block.
Page 3 will have the details listed that you will need as you create this experiment using the paradigm wizard. Feel free to use the previous tutorial on using E-Studio and the Wizard
Details
We will be using the Paradigm Wizard to allow you to put together an experiment on the number Stroop Effect. You have already been exposed to the Paradigm Wizard, and by using it you can create other experiments just as easily as you did previously.
1. Open the Paradigm Wizard to begin.
2. Select the general paradigm.
3. Fill in the details as appropriate for your experiment
a) 1 instruction screen (10,000ms or space bar)
Tell the participants how to respond to the stimuli. Specify the keys they should use (i.e., “3” if the string length is 3, “4” for a length of four, and so on), give an example of a stimulus they might see and how they would correctly respond to that stimulus. Tell them to hit the space bar to begin the practice session. Be specific, but not too wordy; you only have one instruction screen.
b) 1 goodbye screen (5,000ms or space bar)
c) 1 practice block (so that your subjects can get accustomed to the set up of experiment, computer keys, etc.)
d) 1 experiment block (where the reaction times, etc., will actually be tabulated)
e) The only difference between blocks is whether it is being done in practice mode or not (no other block variables).
f) For the experiment block, you want 20 conditions (“3” x 4 combinations, “4” x 4 combinations, “5” x 4 combinations, “6” x 4 combinations, and four different control combinations – “*” x 4). Make sure the conditions will be presented in random order. Have 2 cycles through all the possible conditions.
g) For the practice block, you'll want 12 trials: just randomly pick 12 cases from the 20 conditions.
h) A trial consists of a fixation (a '+' for 1000 ms), a stimulus display (duration is 10,000ms or subject responds with keys 3456, correct answer in "CorrectAnswer"), and then feedback (for a duration of 2000ms)
Trial Variables:
There are 3 trial “variables” we need to specify:
Condition (which will be either congruent, incongruent, or control)
Stimulus (the 20 possible combinations that will be the stimuli)
Correct Answer (how the subject should respond)
Extra note: You must weight the congruent and control conditions. We must weight the congruent and control conditions because there are only 4 possible combinations in each, whereas for the incongruent condition, there are 12 possible combinations. By weighting all the congruent and control stimuli as 3, we will have an equal amount of trials that are congruent (4 x 3 = 12), incongruent (12), and control (4 x 3 = 12). Go to the “Weight” column (on the far left) and enter 3 for each of the congruent and control conditions (but NOT the incongruent condition).