1. Teacher
Introductory Statistics
Lesson 2.4 D
Objective: SSBAT interpret standard deviation and use the Empirical Rule.
Standards: S2.5B

2. Describes how spread out or scattered the data is
Review
Variation of Data
Describes how spread out or scattered the data is
Measures of Variation
Range
Variance
3. Standard Deviation

3. Deviation of an Entry in a population
Review
Deviation of an Entry in a population
The Difference between an entry in the data set (x) and the mean of the data set
Deviation of x = x – 𝝁

4. Salary (thousands $) x
Deviation
x – 41.5 41 38 39 45 47 44 37 42
-0.5
-3.5
-2.5
3.5
5.5
2.5
-4.5
0.5

5. Deviation is the distance each entry is away from the Mean
Deviation is the distance each entry is away from the Mean. We could not average the deviations together because the sum will always be zero.
Therefore, we squared each deviation and then found the mean of these values – This was referred to as the Variance
Then, we square rooted the variance to come up with the Standard Deviation – This gave us the typical amount an entry deviates from the mean.

6. Standard Deviation  𝝈 or s
A measure of the Typical amount an entry deviates from the Mean
(The typical difference an entry is away from the mean)
The more the entries are spread out, the greater the standard deviation
When all the data entries are the Same, the standard deviation is 0

7. Bell Shape Curve (Normal Distribution)
 The distribution is symmetrical

8. Teacher
Empirical Rule
For data with a bell-shape (symmetrical) distribution, the Standard Deviation has the following characteristics:
 About 68% of the data lie within ONE standard deviation of the mean (1 on each side of the mean)
 About 95% of the data lie within TWO standard deviations of the mean (2 on each side of the mean)
 About 99.7% of the data lie within THREE standard deviations of the mean (3 on each side of the mean)

9. Empirical Rule

10. Empirical Rule – Another way to look at it

11.  99.7% of the data lies within 3 standard deviations of the mean
Examples Using Empirical Rule
The mean rate for satellite television from a sample of households was $49.00 per month, with a standard deviation of $2.50 per month. Between what 2 values does 99.7% of the data lie?
 99.7% of the data lies within 3 standard deviations of the mean
 Therefore, Add 2.50 three times to the mean and then Subtract it three times from the mean to get the two endpoints

12. Continued
The mean rate for satellite television from a sample of households was $49.00 per month, with a standard deviation of $2.50 per month. Between what 2 values does 99.7% of the data lie?
 = 56.5
 – 2.5 – 2.5 – = 41.5
 Therefore 99.7% of the data lies between $ and $56.50.

13. SAT verbal scores in a particular year were normally distributed (bell shaped) with a Mean of 489 and Standard Deviation of 93. Between what two values does 68% of the data lie?
 68% of the data lies within 1 standard deviation of the mean
 Therefore, Add 93 one time to the mean and then Subtract it one time from the mean to get the two endpoints

14. Therefore, 68% of the data lies between 396 and 582.
Continued
SAT verbal scores in a particular year were normally distributed (bell shaped) with a Mean of 489 and Standard Deviation of 93. Between what two values does 68% of the data lie?
 = 582
489 – 93 = 396
Therefore, 68% of the data lies between 396 and 582.

15. Complete Worksheet 2.4 D