DATA ANALYSIS

GRAPHS• Graphs are easy to read, and highlight

distribution’s shape. The are useful because they show the full range of variation and identity data anomalies that might be in need of further study.

• Most common are bar charts, histograms, and frequency polygon.

• Bar chart – contains solid bars separated by spaces. It is a good tool for displaying the distribution of variables measured at the nominal level and other discrete categorical variables. There is a gap between each of the categories.

• Histograms – bars are adjacent, are used to display the distribution of quantitative variables that vary along a continuum that has no necessary gaps.

AGE OF RESPONDENT

90.0

85.0

80.0

75.0

70.0

65.0

60.0

55.0

50.0

45.0

40.0

35.0

30.0

25.0

20.0

200

100

0

Std. Dev = 17.03

Mean = 44.5

N = 1422.00

• Frequency Polygon—continuous line connects the points representing the number or percentage of cases with each value. This is an alternative to the histogram when the distribution of quantitative, continuous variable must be displayed.

Important Guidelines Regarding Graphs • Begin the graph of a quantitative variable at

0 on both axes. • Always use bars of equal width.• The two axes (X and Y) should be of

approximately equal length. • Avoid chart junk that can confuse the reader

and obscure the distribution’s shape.

• Graphs should contain labels, titles and number e.g. Fig. 1. Bar char showing gender distribution.

FREQUENCY DISTRIBUTION

• A frequency distribution displays the number, percentage (the relative frequencies), or both of cases corresponding to each of a variable’s values or group of values.

Death Penalty Statutes 1993

Source: Kathleen Maguire and Ann L. Pastore, eds., Sourcebook of Criminal Justice Statistics. 1994. U.S. Department of Justice, Bureau of Justice Statistics. Washington, D.C.: U.S. Government Printing Office, 1995, pp. 115-116.

StateMinimumAge State

MinimumAge

Arkansas 14 Texas 17

Virginia 15 California 18

Alabama 16 Colorado 18

Delaware 16 Connecticut 18

Indiana 16 Illinois 18

Kentucky 16 Louisiana 18

Mississippi 16 Maryland 18

Missouri 16 Nebraska 18

Nevada 16 New Jersey 18

Oklahoma 16 New Mexico 18

Wyoming 16 Ohio 18

Georgia 17 Oregon 18

New Hampshire 17 Tennessee 18

North Carolina 17

Frequency

1

1

9

4

12

Total N 27

Creating a Frequency Distribution

Minimum Age Tally

14 |

15 |

16 |||||||||

17 ||||

18 ||||||||||||

Creating a Frequency Distribution

Minimum Age Frequency

14 1

15 1

16 9

17 4

18 12

Total N 27

• The components of the frequency distribution should be clearly labeled, with a title, a stub (labels for values of the variable), a caption (identifying whether the distribution includes frequencies, percentages or both).

• Frequency distribution can provide more precise information than a graph about the number and percentage of cases in a variable’s categories.