1. The Normal Distribution
The Normal Curve
It’s a frequency distribution that often occurs when there is a large number of values in a data set.
 The graph is a symmetric, bell-shaped curve known as the normal curve.
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 Most of the data occurs around the mean.
 A small portion of the data occurs in the tails.

2. Properties of a Normal Distribution
The Normal Distribution
Properties of a Normal Distribution
•The maximum point of the curve is at the mean.
•The curve extends indefinitely far to the left and right and approaches the x-axis.
•With a large standard deviation the curve will be flat.
•With a small standard deviation the curve will be tall.
•The total area under the curve is 1.
•The curve is symmetric about the mean.
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•About 68% of the data are within 1 standard deviations from the mean.
•About 95% of the data are within 2 standard deviations from the mean.
•About 99.7% of the data are within 3 standard deviations from the mean.

3. No matter the shape of the bell curve, the area under it is the same.
The Normal Distribution
No matter the shape of the bell curve, the area under it is the same.
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4. The Normal Distribution
Example
Suppose the mean of a set of data is 60 and the standard deviation is 6.
68%
Boundaries: to to 54 to 66
68% of the values in this set of data lie within 1 standard deviation of 60, that is, between 54 and 66.
Print this page for the students. 54 60 66
If you randomly select one item from the sample, the probability that the one you pick will be between 54 and 66 is 0.68.
If you repeat this process 1000 times, approximately 68% of those selected will be between 54 and 66.

5. The Normal Distribution
Ex. 1
The Normal Distribution
The lifetimes of 10,000 watch batteries are normally distributed.
The standard deviation is 60 days.
The mean is 500 days.
Sketch a normal curve that represents the frequency distribution of lifetimes of the batteries.
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6. The Normal Distribution
Ex. 2
The Normal Distribution
Suppose the scores of 500 college freshmen taking Psychology 101 are normally distributed.
The standard deviation is 10.
The mean score is 60%
Sketch a normal curve that represents the frequency distribution of scores.
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7. The Normal Distribution
Ex. 3
The Normal Distribution
Find the upper and lower limits of the interval about the mean in which 68%, 95%, and 99.7% of the values of a set of normally distributed data can be found if the mean is 124 and σ is 16.
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8. The Normal Distribution
Ex. 4
The lifetimes of 10,000 watch batteries are normally distributed.
The standard deviation is 60 days.
The mean is 500 days.
How many batteries will last between days?
 How many batteries will last between days?
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 How many batteries will last between days?

9. More Examples
What percent of batteries will last between 500 and 560 days?
What percent of batteries will last longer than 620 days?
What percent of batteries last 320 days at the most?