Chapter 13 Notes (Student)
1
Normal Distribution
• Strategy for Exploring Data for One Quantitative
Variable:
– Always plot the data: Histogram or stemplot
– Look for the overall pattern and for striking deviations such
as outliers.
– Describe the center and the spread with the median and
quartiles or the mean and the standard deviation.
New Step
– Sometimes the overall pattern of a large number of
observations is so regular that we can describe it by a
smooth curve.
Density Curves
• Density Curve
– a curve that is superimposed on a
relative frequency histogram to outline the shape.
• The area under the density curve is 1.
• Easy and quick way of describing the shape of a
distribution.
Density Curve
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2
Using a Density Curve
• Histograms show the frequency or relative frequency
of observations in each class interval.
• Density curves show the proportion of observations
in any region by areas under the curve.
Example:
We have a data set of 1000 sample proportions.
What proportion of the observations are greater than 0.61?
Using a Density Curve
Center and Spread of a Density Curve
•M
e
d
i
a
n
– the point that divides the area under the
curve in half.
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 Fall '07
 JOHNSON
 Statistics, Normal Distribution, Standard Deviation, Andy, Midwest school

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