How to ensure a correct number of trials

I am coding a 2-back task with reference to the one provided by the psytoolkit experiment library. I want to ensure that among the 55 trials of the nback_real1 block, exactly 15 of them are correct, matching, 2-back trials. May I know how to do so?

Below is my code

options

bitmapdir stimuli

set &stimulus_display_time 500 # stimulus presentation

set &iti 2500 # intertrial interval

set &choosechance 3 # 1 in "choosechance" are matching. The higher the number, the fewer matches



fonts

arial 18



bitmaps

letterA ## 15 letters

letterB

letterC

letterD

letterE

letterH

letterI

letterK

letterL

letterM

letterO

letterP

letterR

letterS

letterT

overlay

error_feedback

correct_feedback

grey_feedback

instruction1

instruction2

realblock1

realblock2


part check_response1

if $requiredresponse == 0 and STATUS == TIMEOUT

set $score 1 ## so far so good

fi

if $requiredresponse == 0 and STATUS != TIMEOUT

set $score 0 ## wrongly pressed during letter presentation.

set $false_alarm 1

fi

if $requiredresponse == 1 and STATUS != TIMEOUT

set $score 1 ## correctly pressed during letter presentation.

set $match 1

fi




task twoback

keys m

set &trialcount increase

set $currentletter random 1 15 ## choose random letter out of the 15 options

##

set $memory random 1 &choosechance ## random number for choosing condition

##

if $memory == 1 and &trialcount > 2

set $currentletter &nback2

set $requiredresponse 1 ## the m key needs to be pressed later, m=stands for Memory

set $typeoftrial 1

fi

##

if $memory != 1 or &trialcount <= 2 ## chose a letter but not that of 3 trials ago

set $currentletter random 1 15

while $currentletter == &nback2 ## choose anything but NOT that of 3 back

set $currentletter random 1 15

while-end

set $requiredresponse 0 ## no key should be pressed

set $typeoftrial 0

fi

##

draw off

show bitmap $currentletter ## stimulus 1

draw on

readkey 1 &stimulus_display_time

set $extrawait expression &stimulus_display_time - RT

##

set $score 0 ## this is the default, assume error

part check_response1

##

set $my_rt RT

##

delay $extrawait

clear 1

##

if STATUS == TIMEOUT ## means people did not respond yet

readkey 1 &iti

set $my_rt expression RT + &stimulus_display_time

fi

part check_response1

set $extrawait expression &iti - RT

##

if $requiredresponse == 1 and $score == 0

set $miss 1

fi

delay $extrawait

##

set &nback2 &nback1

set &nback1 $currentletter

##

save BLOCKNUMBER &trialcount $typeoftrial $score $match $miss $false_alarm $my_rt $memory $currentletter &nback1 &nback2




block nback_real1

set &trialcount 0 # make sure you use this again if you have another block

message realblock1

tasklist

twoback 55

end