Counterbalancing

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.

Shuffling image position

In the current experimental design, the plural image is always printed on the left and the singular image is always printed on the right.

It’s possible that participants are simply always faster at choosing images on the left (or on the right). This reaction time advantage would create a confound that could lead to an incorrect conclusion about morphological processing.

 
@// code omitted in interest of space
@
@// Experimental trial
*Template("items.csv", row => 
@    newTrial("experimental-trial",
@        newTimer("break", 1000)
@            .start()
@            .wait()
@        ,
@        newAudio("audio", row.audio)
@            .play()
@        ,
@        newTimer("timeout", row.duration)
@            .start()
@        ,
@        newText("sentence", row.sentence)
@            .center()
@            .unfold(row.duration)
@        ,
@        newImage("plural", row.plural_image)
@            .size(200, 200)
@        ,
@        newImage("singular", row.singular_image)
@            .size(200, 200)
@        ,
@        newCanvas("side-by-side", 450,200)
@            .add(  0, 0, getImage("plural"))
@            .add(250, 0, getImage("singular"))
@            .center()
@            .print()
@            .log()
@        ,
@        newSelector("selection")
@            .add(getImage("plural"), getImage("singular"))
+            .shuffle()
@            .keys("F", "J")
@            .log()
@            .callback(getTimer("timeout").stop())
@        ,
@        getTimer("timeout")
@            .wait()
@        ,
@        getAudio("audio")
@            .stop()
@    )
@    .log("group", row.group)
@    .log("item", row.item)
@    .log("condition", row.inflection)
@)
@
@// code omitted in interest of space

The shuffle command shuffles the image positions for every "experimental-item" trial independently, meaning that within an iteration of the AdvancedTutorial experiment it’s still possible for the plural image to be printed on the left for all four experimental items.

Another way of counterbalancing is to explicitly set half of the items to display the plural image on the left, and the other half to display the plural image on the right. In this experiment, we use the shuffle command, but include the hardcoded counterbalancing for reference:

 
@// code omitted in interest of space
@
@// Experimental trial
*Template("items.csv", row => 
@    newTrial("experimental-trial",
@        newTimer("break", 1000)
@            .start()
@            .wait()
@        ,
@        newAudio("audio", row.audio)
@            .play()
@        ,
@        newTimer("timeout", row.duration)
@            .start()
@        ,
@        newText("sentence", row.sentence)
@            .center()
@            .unfold(row.duration)
@        ,
!        newImage("left", row.left_image)
@            .size(200, 200)
@        ,
!        newImage("right", row.right_image)
@            .size(200, 200)
@        ,
@        newCanvas("side-by-side", 450,200)
!            .add(  0, 0, getImage("left"))
!            .add(250, 0, getImage("right"))
@            .center()
@            .print()
@            .log()
@        ,
@        newSelector("selection")
!            .add(getImage("left"), getImage("right"))
@            .keys("F", "J")
@            .log()
@            .callback(getTimer("timeout").stop())
@        ,
@        getTimer("timeout")
@            .wait()
@        ,
@        getAudio("audio")
@            .stop()
@    )
@    .log("group", row.group)
@    .log("item", row.item)
@    .log("condition", row.inflection)
@)
@
@// code omitted in interest of space

Randomizing item order

By default, PennController executes experiment scripts from top to bottom. The global command Sequence, which is a handler for the Ibex shuffleSequence variable, allows you to specify a shuffle sequence instead.

A shuffle sequence is a way to sequence or randomize an array of items, in this case the PennController trials. Randomizing the order of the experimental items is a way to control for order effects.

The Sequence global command can create a wide variety of shuffle sequences using type predicates (predicates, for short), and the functions seq, randomize, shuffle, and rshuffle.

For the AdvancedTutorial experiment, the important parts to know are that:

• Predicates can be trial labels.
• Sequence(PREDICATE_1, PREDICATE_2, ...) creates a sequence in which all trials that match PREDICATE_1 precede all trials that match PREDICATE_2, and so on.
• randomize(PREDICATE) creates a sequence in which all trials that match PREDICATE are randomly ordered.
• Trials that do not match a PREDICATE are not run.

 
@// Type code below this line.
@
@// Remove command prefix
@PennController.ResetPrefix(null)
@
@// Turn off debugger
@// DebugOff()
@
+// Control trial sequence
+Sequence("instructions", randomize("experimental-trial"), "send", "completion_screen")
@
@// code omitted in the interest of space

Controlling group assignment

PennController uses an internal counter to counterbalance group assignment. By default, the counter increases by one at the end of an experiment, which rotates which experimental item group is run.

Other options for controlling group assignment:

• Use the global command SetCounter to manually control the internal counter.
• Modify the experiment URL to force assignment to a specific group.

To modify the experiment URL, replace experiment.html with server.py?withsquare=N, where N is a number from 0 to number-of-groups-minus-one. This method is useful if you are sending the URL around for data collection but want to control which group your participants end up in.

For example, the AdvancedTutorial experiment has two groups.

• If a participant clicks the default URL https://expt.pcibex.net/ibexexps/angelicapan/AdvancedTutorial/experiment.html, then they will be assigned to group A or B depending on the internal counter.
• If a participant clicks the link https://expt.pcibex.net/ibexexps/angelicapan/AdvancedTutorial/server.py?withsquare=0, then they will be assigned to group A.
• If a participant clicks the link https://expt.pcibex.net/ibexexps/angelicapan/AdvancedTutorial/server.py?withsquare=1, then they will be assigned to group B.