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Basic Descriptive Statistics Percentages and Proportions Ratios and Rates Frequency Distributions: An Introduction Frequency Distributions for Variables Measured at the Nominal and Ordinal Levels

Basic Descriptive Statistics (cont) Frequency Distributions for Variables Measured at the Internal Ratio Levels Constructing Frequency Distributions for Interval-ratio Level Variables Charts and Graphs Interpreting Statistics: Analyze Changing Patterns of Workplace Surveillance

Percentage The number of cases in a category of a variable divided by the number of cases in all categories on the variable, the entire quantity multiplied by 100. Proportion The number of cases in one category of a variable divided by the number of cases in all categories of the variable.

ratios The number of cases in one category divided by the number of cases in some other category. rate The number of actual occurrences of some phenomena or trait divided by the number of possible occurrences per some unit of time.

frequency distributions Tables that summarize the distribution of a variable by reporting the number of cases contained in each category of the variables. class intervals Categories used in the frequency distributions for interval-ratio variables.

midpoints The points exactly halfway between the upper and lower limits, found by dividing the sum of the upper and lower limits by two. cumulative frequency An optional column in a frequency distribution that displays the number of cases within an interval and all preceding intervals.

cumulative percentage An optional column in a frequency distribution that displays the percentage of cases within an interval and all preceding intervals. pie charts A graphic display device especially for discrete variables with only a few categories.

bar charts A graphic display device for discrete variables. histogram A graphic display device for interval-ratio variables. Class intervals are represented by contiguous bars of equal width, the height of each corresponding to the number of cases in the interval.

line charts A graphic display device for interval-ratio variables. Class intervals are represented by dots placed over the midpoint, the height of each corresponding to the number of cases in the interval. All dots are connected by straight lies. graphic display device for interval-ratio variables. Class intervals are represented by dots placed over the midpoint, the height of each corresponding to the number of cases in the interval. All dots are connected by straight lies.

Deciding on a Descriptive Format How to present the data. What kind of information to lose. How much detail can safely be obscured.

Guidelines for Using Percentages and Proportions 1. When working with a small number of cases, report the actual frequencies rather than percentages of proportions. 2. Always report the number of observations along with proportions and percentages. 3. Proportions and percentages can be calculated at any level of measurement.

Graphing Techniques: Discrete Variables Pie charts Bar charts

Graphing Techniques: Interval-ratio Variables Histogram Line charts

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