文档介绍：Describing Data: Frequency Tables, Frequency Distributions, and Graphic Presentation
Chapter 2
GOALS
Organize qualiarts. This is the average of the upper and lower class limits.
Class frequency: The number of observations in each class.
Class interval: The class interval is obtained by subtracting the lower limit of a class from the lower limit of the next class.
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EXAMPLE – Creating a Frequency Distribution Table
Ms. Kathryn Ball of AutoUSA wants to develop tables, charts, and graphs to show the typical selling price on various dealer lots. The table on the right reports only the price of the 80 vehicles sold last month at Whitner Autoplex.
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Constructing a Frequency Table - Example
Step 1: Decide on the number of classes.
A useful recipe to determine the number of classes (k) is the “2 to the k rule.” such that 2k > n.
There were 80 vehicles sold. So n = 80. If we try k = 6, which means we would use 6 classes, then 26 = 64, somewhat less than 80. Hence, 6 is not enough classes. If we let k = 7, then 27 128, which is greater than 80. So the recommended number of classes is 7.
Step 2: Determine the class interval or width.
The formula is: i (H-L)/k where i is the class interval, H is the highest observed value, L is the lowest observed value, and k is the number of classes.
($35,925 - $15,546)/7 = $2,911
Round up to some convenient number, such as a multiple of 10 or 100. Use a class width of $3,000
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Step 3: Set the individual class limits
Constructing a Frequency Table - Example
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Step 4: Tally the vehicle selling prices into the classes.
Step 5: Count the number of items in each class.
Constructing a Frequency Table
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Relative Frequency Distribution
To convert a frequency distribution to a relative frequency distribution, each of the class frequencies is divided by the total number of observations.
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Graphic Presentation of a Frequency Distribution
The three commonly used graphic forms are:
Histograms
Frequency