Supporting Data for 2017 DPIC Study of Murder Rates and Killings of Police

How DPIC conducted the analysis

Our Goal

We want­ed to find out what homi­cide num­bers and trends would tell us about whether the death penal­ty deterred mur­ders and, more specif­i­cal­ly, what kind of impact — if any — it had in deter­ring killings of law enforce­ment in the line of duty.

How We Classified the States

We broke the states down into three cat­e­gories that we thought would tell us some­thing dif­fer­ent about whether the death penal­ty was a deter­rent and cre­at­ed four comparison groups:

1. United States: the coun­try as a whole.
2. Death-Penalty States: those states that have had the death penal­ty con­tin­u­ous­ly from 2000 onward (death penalty states).
3. Non-Death-Penalty States: those states that have nev­er had the death penal­ty at any time in the 21^st century.
4. Transitional States: those states that had the death penal­ty as of 2000 but, as a result of leg­isla­tive or judi­cial abo­li­tion, no longer do.

Hypotheses

Our hypothe­ses were that, if the death penal­ty were a deter­rent, then — all oth­er things being equal:

• Murder rates should be low­er in states with the death penal­ty than in states that did not autho­rize capital punishment.
• When homi­cide rates rise nation­al­ly, they should rise less in death-penal­ty states than in non-death-penalty states.
• When homi­cide rates fall nation­al­ly, they should fall more in states that have the death penal­ty than in states that don’t.
• Murder rates should rise more (or fall less) after states abol­ish the death penal­ty in com­par­i­son to both death-penal­ty states and states that had long before abol­ished the death penalty.
• The pat­tern of dis­pro­por­tion­ate­ly large increase and dis­pro­por­tion­ate­ly small decrease in the tran­si­tion­al states should be con­sis­tent across the states that abol­ish the death penalty.

With respect to whether the death penal­ty pro­tects law enforce­ment, we hypoth­e­sized that every­thing that should be true about the death penal­ty in gen­er­al should also be true with respect to the killings of police offi­cers. if the death penal­ty were nec­es­sary to pro­tect law enforce­ment, then — all oth­er things being equal:

• The rates at which law enforce­ment per­son­nel were mur­dered should be low­er in states with the death penal­ty than in states that did not autho­rize capital punishment.
• When killings of police offi­cers rise nation­al­ly, they should rise less in death-penal­ty states than in non-death-penalty states.
• When rates of killing offi­cers fall nation­al­ly, they should fall more in states that have the death penal­ty than in states that don’t.
• In tran­si­tion­al states that abol­ish the death penal­ty, there should be a notice­able change in the rates at which police were killed, and the rates of police killings should not only rise fol­low­ing abo­li­tion of the death penal­ty, but should rise dis­pro­por­tion­ate­ly to the rates at which law enforce­ment per­son­nel are killed both in long-time death-penal­ty states and in states that had long ago abol­ished the death penalty.

In addi­tion, if the death penal­ty was more nec­es­sary to pro­tect police offi­cers than oth­er poten­tial vic­tims or had any spe­cial deter­rent val­ue when it came to homi­cides of law enforcement personnel:

• The per­cent­ages of all mur­ders that involve law enforce­ment vic­tims should be low­er in death penal­ty states than in states that do not have the death penalty.
• If police are espe­cial­ly vul­ner­a­ble with­out the death penal­ty, mur­ders of offi­cers should rise as a per­cent­age of all homi­cides when states abol­ish the death penalty.
• If there are increas­es in mur­ders of police offi­cers, the pat­tern of increas­es should be con­sis­tent across the states that have abol­ished the death penalty.

The Data We Used

We looked at mur­der rates nation­al­ly from 1987 through 2015. We chose 1987 as the start­ing date, because that was the ear­li­est date for which we found FBI Uniform Crime Statistics on offi­cers felo­nious­ly killed in the line of duty.

• Our source of infor­ma­tion on the num­ber of mur­ders nation­wide and in each state was the FBI Uniform Crime Reports, Annual Murder Data from 1987 to 2015.
• Our source of infor­ma­tion on the num­bers of mur­ders of law enforce­ment nation­wide and in each state was the FBI Uniform Crime Reports, Law Enforcement Officers Killed & Assaulted, Officers Feloniously Killed Annual Data (LEOKA reports), 1987 through 2015.
• Our source of infor­ma­tion on pop­u­la­tion nation­wide and in each state was the FBI Uniform Crime Reports, Annual State Population Data from 1987 through 2015.

How We Calculated Rates

We cal­cu­lat­ed the year­ly mur­der rate per 100,000 pop­u­la­tion for each state by divid­ing the num­ber of mur­ders in that state dur­ing that year by the pop­u­la­tion of that state that year and mul­ti­ply­ing by 100,000.

We cal­cu­lat­ed the mur­der rates for 100,000 pop­u­la­tion each year for each of our cat­e­gories of states — Death-Penalty States (“DPS”), Non-Death-Penalty States (“NDPS”), and Transitional States by adding the num­ber of mur­ders for all of the states in each cat­e­go­ry that occurred dur­ing a giv­en year and divid­ing it by the com­bined pop­u­la­tion of all of the states in that cat­e­go­ry that year, then mul­ti­ply­ing the result by 100,000.

For the aver­age homi­cide rates for each state and each cat­e­go­ry, we totaled all of the mur­ders in the state or cat­e­go­ry of states over the 29-year peri­od and divid­ed by 29 to obtain the aver­age num­ber of mur­ders. We then totaled the pop­u­la­tion num­bers for each of the 29 years to obtain the aver­age pop­u­la­tion. We then divid­ed the aver­age num­ber of mur­ders by the aver­age pop­u­la­tion and mul­ti­plied by 100,000 to deter­mine the aver­age mur­der rate per 100,000 pop­u­la­tion for the state or cat­e­go­ry of states.

We fol­lowed the same pro­ce­dure in cal­cu­lat­ing the rates at which law enforce­ment offi­cers were felo­nious­ly killed in the line of duty for each state and each cat­e­go­ry of states. Because the num­bers of mur­ders in which law enforce­ment offi­cers were vic­tims was so small, we would have intro­duced math­e­mat­ic errors had we round­ed their totals and cal­cu­lat­ed them based upon mur­ders per 100,000 pop­u­la­tion. To avoid round­ing errors, we cal­cu­lat­ed the num­bers as mur­ders per 10,000,000 pop­u­la­tion. Then, because peo­ple find it eas­i­er to con­ceive of com­par­isons at the 1 mil­lion lev­el rather than at the 10 mil­lion lev­el, we moved the dec­i­mal point one spot to the left so the num­bers reflect the num­ber of mur­ders of law enforce­ment per 1,000,000 population.

Nebraska arguably could fall in the Transitional State cat­e­go­ry, but because its leg­isla­tive abo­li­tion of the death penal­ty was sus­pend­ed and then over­turned by vot­er ref­er­en­da, it nev­er actu­al­ly abol­ished cap­i­tal pun­ish­ment in the state. We have there­fore includ­ed it as a Death-Penalty State.

The Numbers

The num­bers for each state and the cal­cu­la­tions for each cat­e­go­ry of state are con­tained in a series of Microsoft Excel spread­sheets. You can down­load the spread­sheets con­tain­ing the data on which the study is based, and our cal­cu­la­tions from that data, here: https://​death​penal​ty​in​fo​.org/​f​i​l​e​s​/​s​p​r​e​a​s​h​e​e​t​/​A​p​p​e​n​d​i​x​I​T​a​b​l​e​s​.xlsx

Here are the cumu­la­tive totals over the 29-year-peri­od, 1987 to 2015, for each cat­e­go­ry of infor­ma­tion we col­lect­ed. Where we have cal­cu­lat­ed mur­der rates, we have shown the raw num­bers approx­i­mate­ly 14 dec­i­mal places to the right so that you can use these num­bers for your own cal­cu­la­tions with­out wor­ry­ing about round­ing errors. Or, you can take the data from the spreadsheets themselves. 

Total # of Murders

Total # of Murders, United States, 1987 – 2015 = 526,064
Total # of Murders, Death Penalty States, 1987 – 2015 = 374,814
Total # of Murders, Non-Death Penalty States, 1987 – 2015 = 51,528
Total # of Murders, Transitional States, 1987 – 2015 = 99,722

Source: FBI Uniform Crime Reports, Annual Murder Data

Total # of Police Officers Killed
Total # of Police Officers Killed, United States, 1987 – 2015 = 1,599
Total # of Police Officers Killed, Death Penalty States, 1987 – 2015 = 1,227
Total # of Police Officers Killed, Non-Death Penalty States, 1987 – 2015 = 171
Total # of Police Officers Killed, Transitional States, 1987 – 2015 = 201

Source: FBI Uniform Crime Reports, Law Enforcement Officers Killed & Assaulted, Officers Feloniously Killed Annual Data (LEOKA reports, 1987 through 2015)*

Average Population from 1987 – 2015

Average Population, United States, 1987 – 2015 = 282,401,655.034
Average Population, Death Penalty States, 1987 – 2015 = 194,480,539.034
Average Population, Non-Death Penalty States, 1987 – 2015 = 37,108,178.862
Average Population, Transitional States, 1987 – 2015 = 50,812,937.138

Source: FBI Uniform Crime Reports, Annual State Population Data

Murder Rate/​100K Population

Murder Rate, United States, 1987 – 2015 = 6.424 (6.42352394458151)
Murder Rate, Death Penalty States, 1987 – 2015 = 6.646 (6.64571414385248)
Murder Rate, Non-Death Penalty States, 1987 – 2015 = 4.788 (4.78823709676339)
Murder Rate, Transitional States, 1987 – 2015 = 6.767 (6.76735069621765)

Murder Rate, DP States: 1.03 times high­er than nation­al; 1.39 times high­er than NDP; 1.02 times low­er than Transitional
Murder Rate, NDP States: 1.34 times low­er than nation­al; 1.39 times low­er than DP; 1.41 times low­er than Transitional
Murder Rate, Transitional States: 1.05 times high­er than nation­al; 1.02 times high­er than DP; 1.41 times high­er than NDP

Police Officer Murder Rate/​1M Population

Police Officer Murder Rate, United States, 1987 – 2015 = 0.195 (0.195246486879654)
Police Officer Murder Rate, Death Penalty States, 1987 – 2015 = 0.218 (0.217555674401356)
Police Officer Murder Rate, Non-Death Penalty States, 1987 – 2015 = 0.159 (0.158901673565157)
Police Officer Murder Rate, Transitional States, 1987 – 2015 = 0.136 (0.136402949192731)

Police Officer Murder Rate, DP States: 1.11 times high­er than nation­al; 1.37 times high­er than NDP; 1.59 times high­er than Transitional
Police Officer Murder Rate, NDP States: 1.23 times low­er than nation­al; 1.37 times low­er than DP; 1.16 times high­er than Transitional)
Police Officer Murder Rate, Transitional States: 1.43 times low­er than nation­al; 1.59 times low­er than DP; 1.16 times low­er than NDP

Percentage of Homicides With Officers as Victims

Officer Percentage of Homicides, United States, 1987 – 2015 = 0.304% (0.30395541226923%)
Officer Percentage of Homicides, Death Penalty States, 1987 – 2015 = 0.327% (0.32736237173638%)
Officer Percentage of Homicides, Non-Death Penalty States, 1987 – 2015 = 0.332% (0.33185840707965%)
Officer Percentage of Homicides, Transitional States, 1987 – 2015 = 0.202% (0.20156033773891%)

Officer Percentage of Homicides, DP States: 1.08 times high­er than nation­al; 1.01 times low­er than NDP; 1.62 times high­er than Transitional
Officer Percentage of Homicides, NDP States: 1.09 times high­er than nation­al; 1.01 times high­er than DP; 1.65 times high­er than Transitional
Officer Percentage of Homicides, Transitional States: 1.51 times low­er than nation­al; 1.62 times low­er than DP; 1.65 times low­er than NDP

* See FBI Uniform Crime Reports, Law Enforcement Officers Killed and Assaulted, 2001, n.1 (2001) (“Because a cat­a­stro­phe such as the September 11 attacks falls far out­side the nor­mal course of police expe­ri­ence, the FBI has not includ­ed those fatal­i­ties in the 2001 rate, trend, or dis­po­si­tion tables for to do so would skew the data and ren­der analy­ses mean­ing­less.”), https://​ucr​.fbi​.gov/​l​e​o​k​a​/2001.