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Unit 2 Section 2.1 – Day 2

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2.1: Frequency Distributions and Their Graph Graphs are used to present data after it has been organized into frequency distributions. The purpose of a graphs in statistics is to display data in pictorial form. The three most commonly used graphs are: Frequency Histograms Frequency Polygons Ogives (pronounced: o-jive)

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The Frequency Histogram Frequency Histogram – a bar graph that represents the frequency distribution of a data set. The horizontal scale is quantitative and measures the data entries. The vertical scale measures the frequencies of the class. Consecutive bars must touch. Bars begin and end at class boundaries. Section 2.1

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How to Construct a Frequency Histogram Draw and label the x and y axis Remember x is the horizontal axis and y is the vertical. Represent the frequency on the y axis and the class boundaries on the x axis. Using the frequency as the heights, draw vertical bars for each class. Section 2.1

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Activity: Construct a Histogram Using the data on the following slide, construct a histogram to represent the data. Section 2.1

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Class Limit Class BoundaryFrequency 100-10499.5-104.52 105-109104.5-109.58 110-114109.5-114.518 115-119114.5-119.513 120-124119.5-124.57 125-129124.5-129.51 130-134129.5-134.51 Section 2.1

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The Frequency Polygon Frequency Polygon – a graph that displays 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. Section 2.1

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How to Construct a Frequency Polygon Find the midpoints of each class. Add the upper and lower boundary, then divide by 2. Draw and label the x and y axis Label the x-axis with the midpoint of each class. Determine a suitable scale for the frequencies. Using the midpoints for the x values and the frequencies as the y values, plot the points. Connect the adjacent points with line segments. Draw a line back to the x axis at the beginning and end of the graph. The line should connect at the same distance where the previous and next midpoint would be located. Section 2.1

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Activity: Construct a Frequency Polygon Using the data representing record high data for the 50 states, construct a frequency polygon to represent the data. Section 2.1

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The Ogive Ogive – a graph that represents the cumulative frequencies for the classes in a frequency distribution. Also known as a cumulative frequency graph. Section 2-3

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How to Construct an Ogive Find the cumulative frequency of each class. Draw and label the x and y axis Label the x-axis with the class boundaries of each class. Determine a suitable scale for the frequencies. Plot the cumulative frequency at each upper class boundary. Starting with the first upper class boundary, connect adjacent points. Then extend the graph to the first lower class boundary on the x axis. Section 2.1

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Activity: Construct an Ogive Using the data representing record high data for the 50 states, construct an Ogive to represent the data. Section 2.1

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Relative Frequency Histogram Relative Frequency Histogram – a graph that coverts the distributions from frequencies to proportions of frequencies. To covert, divide the frequency by the overall cumulative frequency. The sum of the relative frequencies will always equal 1. Section 2.1

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Pg. 116 – 117 (1 – 11 ODD) Homework

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