chap02_outline - Chapter 2 Outline Introduction This...

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Chapter 2 Outline Introduction This chapter begins our study of descriptive statistics . Recall from Chapter 1 that when using descriptive statistics we merely describe a set of data. For example, we want to describe the entry-level salary for a select group of professions. We find that the entry-level salary for accountants is $38,000, for systems analysts $48,000, for physician’s assistants $80,000, and so on. This unorganized data provides little insight into the pattern of entry-level salaries, which makes conclusions difficult. This chapter presents a technique that is used to organize raw data into some meaningful form. It is called a frequency distribution . To better understand the main features of the data, we portray the frequency distribution in the form of a frequency polygon, a histogram, or a cumulative frequency distribution. The goal is to make tables, charts, and graphs that will quickly reveal the shape of the data. Constructing a Frequency Distribution A frequency distribution is a useful statistical tool for organizing a mass of data into some meaningful form. Frequency Distribution : A grouping of data into mutually exclusive classes showing the number of observations in each. As noted, a frequency distribution is used to summarize and organize large amounts of data. The steps to follow in developing a frequency distribution are: 1. Decide on the number of classes. 2. Determine the class interval or width. 3. Set the individual class limits. 4. Tally the observations into the appropriate classes. 5. Count the number of items in each class. As an example, the lengths of service, in years, of a sample of seventeen employees are given above. The seventeen observations are referred to as raw data or ungrouped data. To organize the lengths of service into a frequency distribution: 1. We decide to have five classes. 2. We used a class width of 2. 3. We used classes 1 up to 3, 3 up to 5, and so on. Length of Service (in years) 4 32 10 6 6 3 2 10 6 6 5 8 4 8 4 6 2 3 3 7 5 Frequency Distribution Lengths of service Tallies Number of employees 1 up to 3 years // 2 3 up to 5 years ////// 6 5 up to 7 years ///// 5 7 up to 9 years /// 3 9 up to 11 yrs. / 1 Total 17
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4. Tally the lengths of service into the appropriate classes. 5. Count the number of tallies in each class as shown. How many classes should there be? A common guideline is from 5 to 15. Having too few or too many classes gives little insight into the data. A rule for determining the number of classes is shown on the next page. The size of the class interval may be a value such as 3, 5, 10, 15, 20, 50, 100, 1,000, and so on. Class Interval : The size or width of the class . The class interval can be approximated by the formula: Class Interval highest value lowest value Class Interval( ) or number of classes H L i i k - - Where: i is the class interval. H
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chap02_outline - Chapter 2 Outline Introduction This...

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