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As we mentioned before, looking at quantitative data -- numbers that come from measurements -- provides answers to statistical questions. We are mainly concerned with situations where the measurements differ; that is, where there is variation in the data. Our answers to statistical questions must take this variation into account, so we need appropriate tools for describing the differences in measurements. Note 4
One such tool is a graphical representation known as a line plot. In a line plot, we mark each possible value between the minimum and maximum data values and then stack dots above each of these values to represent actual counts. A line plot is sometimes called a dot plot.
Recall the raisin counts for 17 boxes of Brand X raisins:
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Number of Raisins in 17 Half-Ounce Boxes |
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29 |
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27 |
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27 |
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28 |
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31 |
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26 |
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28 |
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28 |
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30 |
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29 |
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26 |
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27 |
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29 |
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29 |
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25 |
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28 |
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28 |
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To constuct a line plot, we'll begin by setting up the horizontal axis for this set of data. Since the lowest (minimum) value is 25 and the highest (maximum) value is 31, we'll display this segment of the number line along the horizontal axis.
Next, for each raisin count, place a dot above its corresponding value on the horizontal axis. For example, to display the count of our first box of Brand X raisins, we put a dot above the number 29.
To complete the line plot, we'll place a dot over the value 27, follow that with another dot over the value 27, and so forth, until there is a dot for each value in the data set.
Use the following Interactive Activity to construct the line plot for this raisin data, or use a piece of paper to add the rest of the data to the line plot we began above.
This activity requires the Flash plug-in, which you can download for free from Macromedia's Web site.
When the line plot is complete, the number of dots above each value indicates the frequency, or the number of times, that this particular raisin count appears in the data.
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