Chapter_II-_Frequency_Distributions_and_Graphs - Chapter II...

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Chapter II: Frequency Distributions and Graphs Outline 1. Introduction 2. Organizing data 3. Histograms, frequency polygons and ogives 4. Other types of graphs 5. Summary Objectives 1. Organize data using frequency distributions 2-1. Introduction When conducting a statistical study, the researcher must gather data fort he particular variable under study. For example, if a researcher wishes to study the number of people who were bitten by poisonous snakes in a specific geographic area over the past several years, he or she would have to gather the data from various doctors, hospitals, or health departments. In order to describe situations, draw conclusions, or make inferences about events, the researcher must organize the data in some meaningful way. The most convenient method of the organizing data is to construct a frequency distribution. After organizing the data, the researcher must present them so they can be understood by those who will benefit from reading the study. The most useful method of presenting the data is by constructing statistical charts and graphs. There are many different types of charts and graphs, and each one has a specific purpose. This chapter explains how to organize data by constructing frequency distributions and how to present the data by constructing charts and graphs. The charts and graphs illustrated here are histograms, frequency polygons, ogives, pie graphs, Pareto charts, and time series graphs. 2-2. Organizing Data There are several ways to collect and organize data. The lesson on measures of central tendency shows an example of using a tally/frequency table. Other ways to organize and collect data are: 1. Circle graphs 2. Broken line graphs 3. Frequency histograms 4. Cumulative frequency histograms 5. Box and whisker plots 6. Scatter plots, and 7. Stem and leaf plots.
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Sometimes when dealing with large sets of data, it is more convenient to observe patterns by grouping the data in intervals called classes and making a frequency distribution. A frequency distribution is a table that shows classes or intervals of data entries with a count of the number of entries in each class. The frequency ƒ of a class is the number of data entries in the class. Each class has a lower class limit which is the least number that can belong to the class and an upper class limit that is the largest number that can belong to a class. For example: in the class of data 12-25, 12 would be the smallest number in the set and 25 would be the largest and the ƒ would be the number of data entries that fall between 12 and 25 inclusive. The class width is the distance between the lower and upper class limits. The difference between the minimum and maximum data entries is called the range . Guidelines to Constructing a Frequency Distribution from a Data Set
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Chapter_II-_Frequency_Distributions_and_Graphs - Chapter II...

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