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**Unformatted text preview: **This smooth line is called a frequency polygon. It forms what is called a
density curve.
There are two important characteristics of all density curves: 1) it is always on or above the x-axis, and
2) it has an area under the curve (and above the x-axis) exactly equal to 1. Both characteristics should make sense. The histogram below displays the distribution of grade point
averages (on a 12-point scale) of a group of students. The y-axis represents counts and counts cannot be
negative. Therefore, it is not possible for bars in a histogram to drop below the x-axis. Also, all of the data
are represented in these bars. Therefore, 100% of the data are contained in the bars.
The density curve for this histogram is given below the histogram. Because it is formed from tracing along
the tops of the bars of the histogram, the density curve never falls below the x-axis. Also, since 100% of
the data are contained in the bars of the histogram, 100% of the data are underneath the density curve
(and above the x-axis). 1...

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