(CBS News) -- CBS News contacted more than 150 police departments in big cities across the country to learn more about their implicit racial bias training as part of a year-long look into policing in America.

The 155 police departments contacted include the largest three cities in each state, based on the 2010 census, as well as the District of Columbia, Ferguson and five additional cities that ranked in their state's top three in the 2018 census estimate. Together, these departments police about a fifth of the U.S. population.

What we found:

  • At least 69% reported they do have implicit racial bias training and 57% of those departments said it was added in the five years since Michael Brown was shot to death by a white police officer in Ferguson, Missouri, sparking months of protests nationwide.
  • About 59% of the agencies that responded to follow-up questions say they do not have a way to measure the success or failure of those programs.
  • More than 70% of the departments we contacted completed our survey and 75% of departments who responded told us that they changed their training practices after controversial use-of-force incidents.
  • Of the departments that reported having implicit bias training, 90% say they make it mandatory.

Below is a state-by-state breakdown of what the departments we contacted said about their implicit racial bias and de-escalation training including the average length of training sessions, frequency of training and when they began offering it.

Alabama

Birmingham

  • Implicit racial bias training since 2015. Mandatory. Training frequency: 16 hours more than once per year. 

  • De-escalation training since 2016. Mandatory. Training frequency: 8 hours once every 2-5 years. 

Montgomery

  • Declined to participate

Huntsville

  • Implicit racial bias training since 2018. Mandatory. Training frequency: 4 hours once per year. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 4 hours once per year. 

Mobile

  • Implicit racial bias training since 2016. Not mandatory. Training frequency: 8 hours once per year. 

  • De-escalation training since 2009. Mandatory. Training frequency: 8 hours once per year. 

Alaska

Anchorage

  • Declined to participate. 

Fairbanks

  • Declined to participate. 

Juneau

  • Declined to participate. 

Arizona

Phoenix

  • Implicit racial bias training since 2018. Mandatory. Training frequency: 8 hours at various frequencies. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 11 hours once per year.

Tucson

  • Implicit racial bias training since 2015. Mandatory. Training frequency: 8 hours at various frequencies. 

  • De-escalation training since 2015. Mandatory. Training frequency: Number of hours varies but occurs annually. 

Mesa

  • Implicit racial bias training since 2018. Mandatory. Training frequency: 10 once every 2-5 years. 

  • De-escalation training since 2016. Mandatory. Training frequency: 10 hours once per year. 

Arkansas

Little Rock

  • Implicit racial bias training since before 2005. Mandatory. Training frequency: 2 hours once per year. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 8 hours once per year. 

Fort Smith

  • Implicit racial bias training since 2016. Mandatory. Training frequency: 4 hours once per year. 

  • De-escalation training since 2015. Mandatory. Training frequency: 4 hours once per year. 

Fayetteville

  • Implicit racial bias training since before 2005. Mandatory. Training frequency: 1 hour once per year. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 1 hour more than once per year. 

California

Los Angeles

  • Declined to participate. 

San Diego

  • Unconscious bias training since 2015. Mandatory. Training frequency: 3 hours once every 2-5 years. 

  • De-escalation training since 2005. Mandatory. Training frequency: Continuous and part of overall training process. 

San Jose

  • Unconscious bias training since 2016. Mandatory. Training frequency: 8 hours at various frequencies. 

  • De-escalation training since before 2005. Mandatory. Training frequency: Number of hours varies, but training occurs more than once per year. 

Colorado

Denver

  • Implicit racial bias training since 2008. Mandatory. Training frequency: 8 hours once every 2-5 years. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 2 hours once every 2-5 years. 

Colorado Springs

  • Declined to participate. 

Aurora

  • Implicit racial bias training since 2010. Mandatory. Training frequency: 2-4 hours once per year. 

  • De-escalation training since 2010. Mandatory. Training frequency: 2-8 hours more than once per year. 

Connecticut

Bridgeport

  • Declined to participate. 

New Haven

  • Did not respond to survey after multiple follow-ups. 

Stamford

  • Unsure if they offer implicit racial bias training. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 8 hours once per year. 

Hartford

  • Implicit racial bias training since 2015. Mandatory. Training frequency: 6 hours at various frequencies. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 4 hours at various frequencies. 

Delaware

Wilmington

  • Did not respond to survey after multiple follow-ups. 

Dover

  • Unconscious bias training since before 2005. Mandatory. Training frequency: One hour once per year. 

  • De-escalation training since 2005. Mandatory. Training frequency: More than two hours once per year. 

Newark

  • Implicit racial bias training since 2012. Mandatory. Training frequency: 2 hours (skipped question regarding frequency of training session). 

  • De-escalation training since before 2005. Mandatory. Training frequency: 2 hours (skipped question regarding frequency of training session). 

District of Columbia

Washington

  • Implicit racial bias training since 2016. Mandatory. Training frequency: 12 hours at various frequencies. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 8 hours more than once per year. 

Florida 

Jacksonville

  • Declined to participate. 

Miami

  • Declined to participate. 

Tampa

  • Implicit racial bias training since July 2014. Mandatory. Training frequency: 4-6 hours once every 2-5 years. 

  • De-escalation training since 2016. Mandatory. Training frequency: No class dedicated to de-escalation; rather, it's included in other training on subjects like use of force and decision making. 

Georgia

Atlanta

  • Implicit racial bias training since 2016. Mandatory. Training frequency: 8 hours for new recruits, and 2 hours for sworn personnel. Occurs once per year. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 8 hours once per year. 

Augusta

  • Implicit racial bias training since 2012. Mandatory. Training frequency: 4 hours at various frequencies. 

  • De-escalation training since 2012. Mandatory. Training frequency: 2 hours more than once per year. 

Columbus

  • Implicit racial bias training since 2016. Mandatory. Training frequency: 2 hours once per year. 

  • De-escalation training since 2017. Mandatory. Training frequency: 2 hours once per year. 

Hawaii

Honolulu

  • Implicit racial bias training since before 2005. Mandatory. Training frequency: Half an hour once per year. 

  • De-escalation training since before 2005. Mandatory. Training frequency: Varies in length but occurs once per year. 

Idaho

Boise

  • Implicit racial bias training since 2018. Mandatory. Training frequency: 4-12 hours at various frequencies. 

  • De-escalation training since 2012. Not mandatory. Training frequency: 24-hour course during police academy, and a 40-hour course after police academy. Occurs at various frequencies. 

Meridian

  • Implicit racial bias training since 2016. Not mandatory. Training frequency: 8 hours at various frequencies. 

  • De-escalation training since 2010. Mandatory. Training frequency: Number of hours varies but occurs more than once per year. Implemented in all of scenario training.  

Nampa

  • Implicit racial bias training since 2008. Mandatory. Training frequency: 4 hours once every 2-5 years. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 40 hours once every 2-5 years. 

Illinois

Chicago

  • Declined to participate

Aurora

  • Implicit racial bias training since before 2005. Mandatory. Training frequency: 8 hours once every 2-5 years. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 8 hours more than once per year. 

Naperville

  • Implicit racial bias training since before 2005. Mandatory. Training frequency: 4 hours once per year. 

  • De-escalation training since 2010. Mandatory. Training frequency: More than 40 hours more than once per year.  

Rockford

  • Implicit racial bias training since 2011. Mandatory. Training frequency: 8 hours once every 2-5 years. 

  • De-escalation training since 2015. Mandatory. Training frequency: 2 hours once per year. 

Indiana

Indianapolis

  • Implicit racial bias training since 2017. Mandatory. Training frequency: 8 hours at various frequencies. 

  • De-escalation training since 2017. Mandatory. Training frequency: Interwoven into firearms curriculum, which is 16 hours per year.  

Fort Wayne

  • Implicit racial bias training since before 2005. Mandatory. Training frequency: 4 hours at various frequencies. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 1 hour more than once per year. 

Evansville

  • Declined to participate

Iowa

Des Moines

  • Implicit racial bias training since 2016, supplemented by annual diversity training. Academy recruits train longer. Mandatory. Training frequency: 2 hours once per year. Academy training is 40 hours.

  • De-escalation training since before 2005. Mandatory. Training frequency: 2 hours at various frequencies. 

Cedar Rapids

  • Implicit racial bias training since 2017. Mandatory. Training frequency: 4 hours once per year. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 4 hours more than once per year. 

Davenport

  • Implicit racial bias training since 2016. Mandatory. Training frequency: 6 hours once per year. 

  • De-escalation training since 2012. Mandatory. Training frequency: 3 hours once per year. 

Kansas

Wichita

  • Implicit racial bias training since 2017. Mandatory. Training frequency: 8 hours at various frequencies. 

  • De-escalation training since 2016. Mandatory. Training frequency: More than 4 hours once per year. 

Overland Park

  • Implicit racial bias training since before 2005. Mandatory. Training frequency: Number of hours varies but training occurs once per year. 

  • De-escalation training since 2018. Mandatory. Training frequency: Number of hours and frequency varies. 

Kansas City

  • Implicit racial bias training since 2015. Mandatory. Training frequency: 2 hours once per year. 

  • De-escalation training since 2017. Mandatory. Training frequency: 2 hours once every 2-5 years. 

Kentucky

Louisville

  • Implicit racial bias training since 2015. Mandatory. Training frequency: 4 hours once per year. 

  • De-escalation training since 2018. Mandatory. Training frequency: 16 hours once per year. 

Lexington

  • Implicit racial bias training since 2016. Mandatory. Training frequency: 8 hours minimum at various frequencies, and 24 hours through police training academy. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 40 hours at various frequencies.  

Bowling Green

  • Declined to participate. 

Louisiana

New Orleans

  • Implicit racial bias training since 2016. Mandatory. Training frequency: 4 hours once per year. 

  • De-escalation training since 2016. Mandatory. Training frequency: Incorporated into use of force classes which occur once per year. 

Baton Rouge

  • Implicit racial bias training since 2017. Mandatory. Training frequency: 8 hours with varying frequency. 

  • De-escalation training since 2005. Mandatory. Training frequency: 8 hours once per year. 

Shreveport

  • Implicit racial bias training since 2018. Not mandatory. Training frequency: 2 hours once per year. 

  • De-escalation training since 2018. Not mandatory. Training frequency: 12 hours once per year. 

Maine

Portland

  • Implicit racial bias training since 2013. Mandatory. Training frequency: 2 hours once every 2-5 years. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 2-3 hours at various frequencies. 

Lewiston

  • Declined to participate 

Bangor

  • Declined to participate. 

Maryland

Baltimore

  • Implicit racial bias training since 2018. Mandatory. Training frequency: In 2018 officers received 2 hours of training. In 2019 officers will receive 32 hours of training. 

  • De-escalation training since 2017. Mandatory. Training frequency: First training sessions are 16 hours. Two-hour refresher courses are given as a follow-up each year.  

Frederick

  • Implicit racial bias training since 2015. Mandatory. Training frequency: Several hours at various frequencies. 

  • De-escalation training since before 2005. Mandatory. Training frequency: Length of training sessions varies, but occurs more than once per year. 

Gaithersburg

  • Implicit racial bias training since before 2005. Mandatory. Training frequency: 1 hour once every 2-5 years. 

  • De-escalation training since 2010. Mandatory. Training frequency: 8-10 hours more than once per year. 

Rockville

  • Implicit racial bias training since 2016. Not mandatory. Training frequency: 1-3 hours at various frequencies. 

  • De-escalation training since 2017. Mandatory. Training frequency: 8 hours at various frequencies. 

Massachusetts

Boston

  • Declined to participate

Worcester

  • Declined to participate

Springfield

  • Implicit racial bias training since 1998. Mandatory. Training frequency: 2-3 hours once every 2-5 years. 

  • De-escalation training since 2014, before Aug. 9. Mandatory. Training frequency: More than one hour of training once per year. 

Michigan

Detroit

  • Implicit racial bias training since before 2005. Mandatory. Training frequency: Included with training (40 hours), which occurs once per year. 

  • De-escalation training since before 2005. Mandatory. Training frequency: Included with training (40 hours), which occurs once per year. 

Grand Rapids

  • Implicit racial bias training since 2016. Mandatory. Training frequency: 8 hours at various frequencies. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 4 hours more than once per year. 

Warren

  • Implicit racial bias training since before 2005. Mandatory. Training frequency: Hours and frequency varies. 

  • De-escalation training since 2015. Mandatory. Training frequency: 4 hours once per year. 

Minnesota

Minneapolis

  • Implicit racial bias training since 2015. Mandatory. Training frequency: 8 hours once per year. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 8 hours more than once per year. 

St. Paul

  • Implicit racial bias training since before 2005. Mandatory. Training frequency depends on the program. Most are 4 hours, more than once per year. 

  • De-escalation training since 2012. Mandatory. Training frequency: 40 hours once per year. 

Rochester

  • Implicit racial bias since 2018. Mandatory. Training frequency: 4 hours once every 2-5 years.

  • De-escalation training since 2005. Mandatory. Training frequency: Between 4 and 40 hours at various frequencies. 

Mississippi

Jackson

  • Implicit racial bias since before 2005. Mandatory. Training frequency: 8 hours once per year. 

  • De-escalation training since before 2005. Mandatory. Training frequency: 8 hours once per year. 

Gulfport

  • Declined to participate

Southhaven

  • Declined to participate

Missouri

Kansas City

  • Implicit racial bias training since 2015. Mandatory. Training frequency: 1-2 hours once per year. 

  • De-escalation training since before 2005. Mandatory. Training frequency: Recruit basic training is continual. In-service training varies, but is more than 4 hours on average each year. 

St. Louis

  • Implicit racial bias training since October 2014. Mandatory. Training frequency: 4 hours once per year. 

  • De-escalation training since October 2014. Mandatory. Training frequency: 4 hours once per year. 

Springfield

  • Declined to participate

Ferguson

  • Implicit racial bias training since 2012. Mandatory. Training frequency: 2 hours once per year. 

  • De-escalation training since 2015. Mandatory. Training frequency: 2 hours once per year. 

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