The National Consortium on Violence Research – NCOVR

Violence in the United States—Data Workbooks
COUNTS AND POPULATION RATES

Two standard measures are commonly used to gauge the volume and risk of crime. Simple crime counts indicate the total volume of crime. Crime counts are important for assessing workload demands on police and other criminal justice agencies.	Crime rates are scaled by population size and provide a measure of the risk of crime—either as offenders or victims—within a population. (“Crime clocks” are another indicator of risk.)

CALCULATE CRIME RATES RATE PER CAPITA is: Crime Rate per Capita = Number of Crimes/Total Population Example: Police reported a total of 16,144 murders to the FBI for year 2002 in a US population of 268,282,065. The murder rate per capita that year was: Murder Rate = 16,144 Murders/268,282,065 US Population = 0.000060 per capita Change the scale units to a RATE PER 100,000 POPULATION by multiplying the per capita rate by 100,000. Murder Rate = 100,000*(16,144 Murders/268,282,065 US Population) = 6.0 per 100,000 Population Read more about police reporting to the FBI.

Population rates are the total count of crimes divided by the size of the population. An annual rate of 3 crimes per 1,000 population, for example, means that there are 3 crimes during a year for every 1,000 persons in the population. By changing the scale units in the denominator, the same rate can also be expressed as 0.003 crimes

per capita or 300 crimes per 100,000 population. The per capita rate indicates the average risk of a crime during a year for each person in the population. Because crime is relatively rare for individuals, crime rates are usually reported as crimes per 100,000 population.

EXPLORING DATA ON AGGRAVATED ASSAULT

Annual data on crimes reported to police are available from the Federal Bureau of Investigation’s Uniform Crime Reports (UCR) program. The data used in this workbook are based on offenses actually reported by police to the FBI and not the FBI’s adjusted estimate of total offenses

that includes estimates for non-reporting agencies. The FBI’s UCR data will be used here to examine variations in crime counts and population rates over time and in different locations within the US.

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The table reports annual average counts of aggravated assaults reported to the FBI by police agencies in different size cities and counties across the US. The counts are sums for all police agencies in a population group and the sums are then averaged over years. Annual averages are available for five-year intervals from 1980 to 1999. During the period from 1980 to 1984 there were an annual average total of 88,255.4 aggravated assaults reported each year in the collection of cities with populations of 1,000,000 or more. The collection of smaller cities with populations under 25,000 reported

an annual average total of 101,102.4 aggravated assaults each year during the same 1980 to 1984 period. By the period 1995 to 1999 the number of aggravated assaults reported by police increased 62% to 142,943.4 annually in the largest cities with populations of 1,000,000 or more compared to an 18% increase to 119,522.0 annually in the smallest cities with populations under 25,000. Print the table of aggravated assault counts for later use.

CHART OF AGGRAVATED ASSAULTS

It is easy to obtain a chart of the annual number of aggravated assaults directly from the table.

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Does the chart display all the population groups on the same screen? If not, you can use chart options to configure the chart on visible screen page. Show Me Each line in chart is a different time period. The legend to the left of the chart identifies the period represented by each line. For example, blue diamonds represent annual average counts in the period 1980-84 and yellow stars represent the period 1995-99. Each marker in a line represents the collection of police agencies from a different population group, starting from very large cities on the left and moving right to smaller cities and then Non-MSA and MSA counties. Both cities and counties are displayed because the main police agency in an area sometimes provides law enforcement services for an entire county, while other areas are policed by city-based agencies.

The vertical axis running up the left side of the chart indicates the annual average total counts in units of 1,000 offenses. For example, the line labeled 100 represents an annual average count of 100,000 (100*1,000) offenses in a year. Counts of aggravated assaults are consistently high for MSA counties and consistently low for Non-MSA counties. Also as was evident in the data table, the annual average count of aggravated assaults was usually highest in the period 1990-94 (red triangles in the chart). Only one population group ever experiences an average close to 200,000 aggravated assaults in a year – cities with populations of 1,000,000 or more during the 1990-94 period.

FACTORS INFLUENCING VARIATIONS IN COUNTS OF AGGRAVATED ASSAULTS

Factors other than crime risk can influence the variation in offense counts across population groups and over time. Simple differences in the size of the population included in the different population groups will affect the counts

even if all population groups face the same risk of crime. It is easy to obtain the population covered by police agencies in each population group.

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If they are not already displayed, restore the percentage change values to the table.

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The table now reports the average annual population covered by police agencies in each population group. The populations policed by law enforcement agencies in MSA counties and cities with populations under 25,000 are largest at about 45-50 million persons annually over the period 1980 to 1999. The next largest population—numbering 25-30 million persons—is policed by law enforcement agencies in Non-MSA counties. Even if the risk of aggravated assault offenses were the same in all population groups, say 300 per 100,000 population, the count of aggravated assaults in MSA counties would be about twice that in cities with populations of 25,000-49,999 persons

and about 4 times the count of aggravated assault offenses in cities with populations between 250,000 and 499,999. These differences in the size of the population at risk could help explain the pattern of higher and lower counts of aggravated assaults observed earlier. For example, a portion of the increase in the count of aggravated assaults over time is likely to be due to the change in the size of the population over the same time periods. Print the population covered table for later use.

VARIATIONS IN AGGRAVATED ASSAULT RATES

Offense rates per population divide the number of offenses by the size of the population. This scales the count of offenses by the size of the population at risk

and provides a more accurate comparison of risk across different size population groups. It is easy to display the aggravated assault rates in each population group.

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The rates in the table show the varying risk of aggravated assaults in different population groups. As expected, when the population covered increases over time, the increases in rates over time are smaller than increases in counts of aggravated assaults. (Compare the percentage change values in the rate table

to the same percentage change values in the aggravated assault count table printed earlier, especially the values for cities with populations over 1 million and cities with populations from 100,000-249,999).

Displaying the data in a chart provides an easy visual comparison of the variation in rates over population groups and over time.

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As in the earlier chart of aggravated assault counts, each line represents a different time period and each marker represents a different population group. After adjusting the count of aggravated assaults by the population covered, the rates per population indicate the overall risk of aggravated assaults (either as offenders or victims)

in different population groups and over time. Higher rates indicate a greater risk of aggravated assaults, while lower rates indicate a lower risk of aggravated assaults for persons residing in places in the different population groups.

The chart is generally ordered by city size and level of urbanization with large cities on the left and then deceasing in size and urbanization as you move to the right. The one exception to this ordering is MSA counties. MSA counties include metropolitan areas in and around cities, while non-MSA counties include rural communities outside metropolitan areas. "Ranking Population Group" Following the general pattern of decreasing aggravated assault rates as size and level of urbanization decline, non-MSA counties have the lowest rate of aggravated assaults—under 200 aggravated assaults per 100,000 population—in all time periods.

These are followed by the smallest cities with populations under 25,000. MSA counties—with substantial portions of their populations residing in suburbs around large cities — have aggravated assault rates that fall between those in cities with populations under 25,000 and cities with populations from 25,000 to 49,999. The one clear exception to a systematic decline in rates as city size and level of urbanization decrease is in cities with populations 500,000-999,999. Rates in these large cities fall between those in cities with populations 250,000-499,999 and cities with populations 100,000-249,999. This exception is evident in all the displayed time periods.

EXPLORING DATA ON ROBBERY

The FBI classifies robbery as a violent crime because it involves the threat or use of force or violence in order to obtain something of value from victims. This offense type also shares elements with theft offenses because it involves the attempt or actual taking of property from another person. The same data that we have been using to examine variations in aggravated assault in the US also contains data

on counts and rates per population for robbery. (Data are also available for other offense types included in the FBI’s category of serious index crimes.) Change the data display from aggravated assault rate chart to “Robbery” count chart.

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Does the chart display all the population groups on the same screen? If not, you can change the window size of chart. Show Me Counts of robbery offenses are consistently high for the largest cities with populations of at least one million and consistently low for Non-MSA counties. Counts of robbery offenses reported to police consistently decline with city size and level of urbanization with distinctive exception for cities between 500,000 and 999,999 population.

Also like aggravated assault counts, the annual average count of robberies was usually highest in the period 1990-94 (red triangles in the chart). However, the closeness of lines in the chart indicates far less variation over time in robbery counts than was evident in aggravated assault counts.