Millisecond Forums

You need to store every trial's payout amount somewhere (in a <list> or <item> element) at runtime and then retrieve one of the entries at random at the end of the experiment. Simple example:

<values>
/ fixedwin = 100
/ randomwin = 0
/ winamount = 0
/ score = 0
</values>

<list randomwins>
/ items = (10, 25, 50, 75, 150, 300, 450, 600)
</list>

<trial choicetrial>
/ ontrialbegin = [values.randomwin=list.randomwins.nextvalue]
/ ontrialend = [if (trial.choicetrial.response=="fixed_amount") values.winamount = values.fixedwin else
    values.winamount = values.randomwin; ]
/ ontrialend = [item.storedwinnings.appenditem(values.winamount); values.score+=values.winamount; ]
/ stimulusframes = [1=question,fixed_amount, random_amount, score]
/ inputdevice = mouse
/ validresponse = (fixed_amount, random_amount)
/ branch = [trial.resulttrial]
</trial>

<trial resulttrial>
/ stimulusframes = [1=winamount, score]
/ validresponse = (0)
/ trialduration = 1000
</trial>

<trial payouttrial>
/ stimulusframes = [1=payoutmsg, payout, trialmsg]
/ validresponse = (57)
</trial>

<block choiceblock>
/ trials = [1-8=choicetrial]
</block>

<block payoutblock>
/ trials = [1=payouttrial]
</block>

<expt>
/ blocks = [1=choiceblock; 2=payoutblock]
</expt>

<item storedwinnings>
</item>

<text fixed_amount>
/ items = ("FIXED WIN")
/ position = (25%, 75%)
</text>

<text random_amount>
/ items = ("RANDOM WIN")
/ position = (75%, 75%)
</text>

<text question>
/ items = ("Would you like to win a FIXED or RANDOM amount?")
</text>

<text score>
/ items = ("Total: <%values.score%>")
/ position = (50%, 25%)
</text>

<text winamount>
/ items = ("You won <%values.winamount%>.")
</text>

<text payoutmsg>
/ items = ("Your payout is")
/ position = (50%, 40%)
</text>

<text payout>
/ items = storedwinnings
</text>

<text trialmsg>
/ items = ("which is the amount you won in trial #<%text.payout.currentitemnumber%>.")
/ position = (50%, 60%)
</text>