Chapter 2
- 1. Chapter 2: Frequency Distributions and Graphs
- 2. Organizing Data Raw data are data in original form. A frequency distribution is the organization of raw data in table form, using classes and frequencies. The frequency is the number of values in a specific class of the distribution. The categorical frequency distribution is used for data that can be placed in specific categories, such as nominal or ordinal data.
- 3. Birth Month - Data September May July October July August May May March September April August June July February January July January November April July March October December July October January December May March March February November April August July October February April June August August November July September December August July September October August December April September September June November November July July December November May August August December January September May
- 5. Grouped Frequency Distribution When the range of the data is large, the data must be grouped into classes that are more than one unit in width. The lower class limit is the smallest data value that can be included in the class. The upper class limit is the largest data value that can be included in the class. The class boundaries are used to separate the classes so that there are no gaps in the frequency distribution.
- 6. Grouped Frequency Distribution The class limits should have the decimal place value as the data, but the class boundaries should have one additional place value and end in a 5. The class width is found by subtracting the lower ( or upper) class limit of one class from the lower ( or upper) class limit of the other class. The class width can also be found by subtracting the lower boundary from the upper boundary. The class width cannot be found by subtracting the limits of a single class.
- 7. Rules for Constructing a Frequency Distribution There should be between 5 and 20 classes. The class width should be an odd number. This ensures that the midpoint of each class has the same place value as the limits. The class midpoint is obtained by adding the lower and upper boundaries and dividing by 2, or adding the lower and upper limits and dividing by 2. The classes must be mutually exclusive. The classes must be continuous. Do not omit classes with a frequency of zero unless they occur at the beginning or ending of the distribution. The classes must be exhaustive. The classes must be equal in width.
- 8. How to Construct a Frequency Distribution To determine the classes Find the highest and lowest values Find the range: Range=R = highest – lowest Width = Range/number of classes (rounded up) Select a starting point for the first class limit. This can be the smallest data value or any convenient number less than the smallest data value.
- 9. Example: Class Age Data Construct a grouped frequency distribution using 7 classes. 23 19 19 22 20 18 32 27 21 22 22 19 25 20 26 19 19 20 21 22 26 21 18 27 19 27 20 20 23 39 30 21 19 20 49 20 34 24 30 24 22 17 20 19 19 28 27 19 27 24 21 21 20 19 34 19 20 22 25 22 20 20 19 20 24 24 26 20 26
- 10. Class Class Classes Boundaries Tally Frequency Frequency 23 19 19 22 20 18 32 27 21 22 22 19 25 20 26 19 19 20 21 22 26 21 18 27 19 27 20 20 23 39 30 21 19 20 49 20 34 24 30 24 22 17 20 19 19 28 27 19 27 24 21 21 20 19 34 19 20 22 25 22 20 20 19 20 24 24 26 20 26
- 11. Cumulative frequencies are used to show how many data values are accumulated up to and including a specific class. When the range of data values is relatively small a frequency distribution can be constructed using single data values for each class. This type of distribution is called an ungrouped frequency distribution .
- 12. Example: BUN Count Example: The blood urea nitrogen (BUN) count of 20 randomly selected patients is given here in milligrams per deciliter. Construct an ungrouped frequency distribution for the data. 17 18 13 14 12 17 11 20 13 18 19 17 14 16 17 12 16 15 19 22
- 13. Reasons For Constructing a Frequency Distribution To organize the data in a meaningful, intelligible way. To enable the reader to determine the nature and shape of the distribution To facilitate computational procedures for measures of average and spread To enable the researcher to draw charts and graphs for the presentation of data. To enable the reader to make comparisons among different data sets.
- 14. 2.3 Histograms, Frequency Polygons, and Ogives The histogram is a graph that displays the data by using contiguous vertical bars (unless the frequency of a class is 0) of various heights to represent the frequencies of the classes. Step 1. Draw and label the x and y axes. Step 2. Represent the frequency on the y axis and the class boundaries on the x-axis. Step 3. Using the frequencies as the heights, draw vertical bars for each class.
- 15. Frequency Polygon The frequency polygon is a graph that displays the data by using lines that connect points plotted for the frequencies at the midpoints of the classes. The frequencies are represented by the heights of the points.
- 16. Ogive The ogive is a graph that represents the cumulative frequencies for the classes in a frequency distribution Step 1. Find the cumulative frequency for each class. Step 2. Draw the x and y axes. Label the x-axis with the class boundaries. Step 3. Plot the cumulative frequency at each upper class boundary .
- 17. Ogive
- 18. Relative Frequency Relative frequency graphs use proportions instead of actual numbers as y-axis values. To convert a frequency into a proportion or relative frequency, divide the frequency for each class by the total of frequencies. The sum of the relative frequencies will always be one. The shape of the graph is the same as those that use frequencies.
- 19. Relative Frequency Relative Cumulative Rel. Cumul. Classes Frequency Frequency Frequency Frequency
- 21. 2.4 Other Types of Graphs A Pareto chart is used to represent a frequency distribution for a categorical variable, and the frequencies are displayed by the heights of vertical bars, which are arranged in order from highest to lowest.
- 23. Time Series Graph A time series graph represents data that occur over a specific time period.
- 24. Example: Time Series Graph Draw a time series graph to represent the data for the number of airline departures (in millions) for the given years. Year 1994 1995 1996 1997 1998 1999 2000 No. of Departures 7.5 8.1 8.2 8.2 8.3 8.6 9.0
- 26. Pie Graph A pie graph is a circle that is divided into sections or wedges according to the percentage of frequencies in each category of the distribution.
- 27. Example: Pie Graph Birth Month Frequency Percentage Degrees January 4 February 3 March 4 April 5 May 6 June 3 July 11 August 9 September 7 October 5 November 6 December 6
- 28. Stem-and-Leaf Plot A stem and leaf plot is a data plot that uses part of the data values as the stem and part of the data values as the leaf to form groups or classes.
- 29. Example: Stem & Leaf Plot The following data represents the number of grams of fat in breakfast meals offered at McDonald’s. Construct a stem-and-leaf-plot. 12 23 28 2 31 37 15 23 38 31 16 11 8 17 20 34 8