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To increase the proportion of "control" (formerly "congruent") trials, it would be easiest to set up a bunch of <list> elements and use those for item selection. Let's look at <trial red> as an example:
<trial red> /ontrialbegin = [values.color = "red"] /stimulustimes = [0=red] /posttrialpause = 250 /validresponse = ("yellow" "green" "blue" "black" "red") /correctresponse = ("red") /ontrialend = [if (text.red.currentitemnumber == 1) { values.count_congruent += 1; values.congruence = 1} else { values.count_incongruent += 1; values.congruence = 2}] /ontrialend = [if (text.red.currentitemnumber == 1 && trial.red.correct) { values.countcorrect_congruent += 1; values.sumrt_congruent += trial.red.latency}] /ontrialend = [if (text.red.currentitemnumber != 1 && trial.red.correct) { values.countcorrect_incongruent += 1; values.sumrt_incongruent += trial.red.latency}]
</trial>
As you can see it displays the stimulus element <text red> with the below items. Whenever the 1st item is selected (the Xs), the trial is recorded as "control" (formerly "congruent") as per the /ontrialend logic highlighted above.
<item colors_red> /1 =" XXXXX " /2 =" green " /3 =" blue " /4 =" black " /5 =" yellow " </item> ... <text red> /items=colors_red /color = (255,0,0) </text>
What that effectively means is that 1 in every 5 instances (20%) of <trial red> will be "control" / "congruent".
Now, let's say you want to increase the proportion of "control" / "congruent" instances to 50% (or 4 out of 8). To do this, you could set up
<list red_items> / items = (1,1,1,1,2,3,4,5) / selectionmode = random </list>
which contains item #1 four times, and use that list for item selection in <text red>:
<text red> /items=colors_red /color = (255,0,0) /select = list.red_items.nextvalue </text>
The changes for the other color <trial> and <text> elements (<trial green> / <text green>, <trial blue> / <text blue>) would be analogous. Only the item numbers differ in each case.
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