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DATA ANALYSIS

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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.

- 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.

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.

Frequency

1

1

9

4

12

Total N27

Minimum AgeTally

14|

15|

16|||||||||

17||||

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

Minimum Age Frequency

141

151

169

174

1812

Total N27

- 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.