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*POLICE OFFICER DILEMMA TASK (PODT)*
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Script Author: Katja Borchert, Ph.D. (katjab@millisecond.com) for Millisecond Software LLC
Date: 04-12-2012
last updated: 02-25-2022 by K. Borchert (katjab@millisecond.com) for Millisecond Software, LLC
Script Copyright © 02-25-2022 Millisecond Software
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BACKGROUND INFO
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This script provides the Police Officer Dilemma Task (PODT); a computer-simulated task
to measure racial bias in shooting decisions.
Correll, J., Park, B., Judd, C.M., & Wittenbrink, B. (2002). The Police Officer's Dilemma:
Using Ethnicity to Disambiguate Potentially Threating Individuals. Journal of Personality
and Social Psychology, 83, 1314-1329. (Experiment 1)
Correll, J., Hudson, S.M., Guillermo, S., & Ma, D.S. (2014). The Police Officer’s Dilemma:
A Decade of Research on Racial Bias in the Decision to Shoot.
Social and Personality Psychology Compass 8/5 (2014): 201–213.
*Adjustments to z-scores (for signal detection parameter calculations) as recommended by:
Gregg, A. & Sedikides, C. (2010). Narcissistic Fragility:
Rethinking Its Links to Explicit and Implicit Self-esteem, Self and Identity, 9:2, 142-161 (p.148)
All images used in this script are the originals. Millisecond Software thanks Dr. Correll et al for
providing generous assistance!
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TASK DESCRIPTION
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Participants see a series of men, either White or Black, that either hold harmless objects or guns,
and have to decide -within a short timeframe- whether to 'shoot' (if there is a gun) or
not to 'shoot' (no gun present). 'Shooting' is done via response button presses.
Before each target appears, participants see a varying number of backgrounds.
The targets (men with objects) appear in the last background shown.
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DURATION
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the default set-up of the script takes appr. 15 minutes to complete
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DATA FILE INFORMATION
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The default data stored in the data files are:
(1) Raw data file: 'podt_raw*.iqdat' (a separate file for each participant)
build: The specific Inquisit version used (the 'build') that was run
computer.platform: the platform the script was run on (win/mac/ios/android)
date, time: date and time script was run
subject, group: with the current subject/groupnumber
session: with the current session id
blockcode, blocknum: the name and number of the current block (built-in Inquisit variable)
trialcode, trialnum: the name and number of the currently recorded trial (built-in Inquisit variable)
Note: trialnum is a built-in Inquisit variable; it counts all trials run; even those
that do not store data to the data file such as feedback trials. Thus, trialnum
may not reflect the number of main trials run per block.
group: 1 = shoot key A; non shoot-key L
2 = shoot key L; non shoot-key A
condition: practice: "gun" vs. "object"
test:
1 = Gun White
2 = Gun Black
3 = Object White
4 = Object Black
itemnumber: itemnumber of the target image
selectbackground: selected background picture is selected
repetition: how many background images are presented
backgroundduration: how long each background slide is presented (in ms)
count_background: counts how many background slides have been presented
response: the participant's response (scancode of response button)
30: A (group1 -> shoot; group2 -> nonshoot)
38: L (group2 -> shoot; group1 -> nonshoot)
responseCat: "shoot", "not shoot", "no response"
correct: the correctness of the response (1 = correct; 0 = incorrect)
result: either "Hit", "Miss", "CR" (correct rejection), "FA" (false alarm) or "no response"
latency: the response latency (in ms); measured from onset of target picture
total: keeps a running tab of the total money earned (or lost)
(2) Summary data file: 'podt_summary*.iqdat' (a separate file for each participant)
inquisit.version: Inquisit version run
computer.platform: the platform the script was run on (win/mac/ios/android)
startDate: date script was run
startTime: time script was started
subjectid: assigned subject id number
groupid: assigned group id number
sessionid: assigned session id number
elapsedTime: time it took to run script (in ms); measured from onset to offset of script
completed: 0 = script was not completed (prematurely aborted);
1 = script was completed (all conditions run)
group: 1 = shoot key A; non shoot-key L
2 = shoot key L; non shoot-key A
total: keeps a running tab of the total money earned (or lost)
ACC_BlackGun: proportion correct shoot decision for Black guys holding a gun
RTcorrect_BlackGun: mean reaction time (in ms) to correctly shoot for Black guys holding a gun
ACC_BlackObject: proportion correct 'non-shoot' decisions for Black guys holding an object
RTcorrect_BlackObject: mean reaction time (in ms) to correctly hit 'not shoot' key for Black guys holding an object
ACC_WhiteGun: proportion correct shoot decision for White guys holding a gun
RTcorrect_WhiteGun: mean reaction time (in ms) to correctly shoot for White guys holding a gun
ACC_WhiteObject: proportion correct 'non-shoot' decisions for White guys holding an object
RTcorrect_WhiteObject: mean reaction time (in ms) to correctly hit 'not shoot' key for White guys holding an object
count_GunBlack-
count_ObjectWhite: counts the trials in each of the 8 conditions
Sum_BlackHits-
Sum_WhiteCRs: keeps a running total of Hits, Misses, FAs, CRs for each ethnicity
MeanRT_BlackHits-
MeanRT_WhiteCRs: the mean reaction times for Hits, Misses, FAs, CRs for each ethnicity (in ms)
percent_BlackHits-
percent_WhiteCRs: the percentages of Hits-Misses and CRs-FAs, separately for each ethnicity
Signal Detection Analysis for hits and false alarms:
dprime: Computes d' (parametric measure of discriminability btw. signals (here: guns) and noise(here: objects))
with d' = z(hitrate) - z(FalseAlarm rate)
=> Range (in this script):
-5.1516586840152740479 <= dprime <= 5.1516586840152740479 (=perfect performance)
=> The higher the value, the better signals (guns) were overall distinguished from noise (objects)
(d' = 0: chance performance; negative d-primes: participant treated nontargets as targets and targets as nontargets)
criterion C: calculates the criterion to 'shoot' vs. not 'shoot'
with criterion C = - (z(hitrate) + z(FalseAlarm rate))/2
* values around 0: no bias towards shooting or not shooting
*the more negative the value, the more lax the criterion to shoot (bias towards shooting)
*the more positive the value, the more stringent the criterion to shoot (bias towards not shooting)
dprime_Black Computes d' for Black targets
criterionC_Black: calculates the criterion to 'shoot' vs. not 'shoot' Black targets
dprime_White Computes d' for White targets
criterionC_White: calculates the criterion to 'shoot' vs. not 'shoot' White targets
Note: Correll et al (2014) report that generally d-prime values tend to be similar between the two groups, showing that
participants are able to distinguish between guns and objects similarly well for Black and White targets.
The criterions 'to shoot', however, tend to differ in that it is more lax ('biased towards shooting')
for Black actors than it is for White actors.
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EXPERIMENTAL SET-UP
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Design: 2 (ethnicity: black vs. white) x 2 (gun vs. no gun) = 4 experimental conditions,
all tested within participants in a mixed design (see Correll et al, 2002)
1 Practice Block: 20 trials
1 Test Block: 80 Trials, 20 trials in each of the 4 conditions, conditions are randomly sampled without replacement
(see list.condition for details)
- shoot keys are counterbalanced by groupnumber
- Number of background images in background slideshow:
randomly (with replacement) determined, between 1-4 (see list.repetition) (-> Correll et al, 2002)
Trials Sequences:
Fixation (500ms)-> Background (variable number) (500-1000ms)-> target (850ms)-> Feedback (3000ms)
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STIMULI
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original pictures; editable under section Editable Stimuli
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INSTRUCTIONS
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Current Instructions modelled after the latest instructions used by Correll et al (these later instructions are
less detailed than the original 2002 instructions)
Instructions are provided by Millisecond Software
as htm/html pages and can be edited by changing the provided htm/html files.
To edit htm/html-files: open the respective documents in simple Text Editors such as TextEdit (Mac)
or Notepad (Windows).
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EDITABLE CODE
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check below for (relatively) easily editable parameters, stimuli, instructions etc.
Keep in mind that you can use this script as a template and therefore always "mess" with the entire code to further customize your experiment.
The parameters you can change are:
HitReward....FAPunish: amounts earned or lost depending on Hits, Misses, Correct Rejections (CR), False Alarms (FA)
fixationduration: the duration of the fixation cross in ms (default: 500, original)
timeout: timeout of main experimental trials in ms, default = 850ms
feedbackduration: the duration of the feedback in ms, default = 3000
key1/key2: the scancodes of the shoot and noshoot keys (keys are counterbalanced), default: "A" and "L"
Note: for the script to run correctly on touchscreen, key1 should be the left key
and key2 should be the right key.
/optionalSummaryFeedback: true (1) = Participants receive a summary feedback matrix (with %correct and incorrect
responses as well as response times for correct choices) (default)
false (0) = no final feedback is provided
