1. Describing Location in a Distribution
Normal Distributions

2. Normal Distribution
Described by a Normal density curve (bell-shaped curves)
Specified by its mean, μ, and standard deviation σ
Always symmetric with the mean at the center, but exact shape depends on μ and σ
Change in curvature (point of inflection) shows where 1 standard deviation from the mean is located.
Abbreviate the Normal distribution as
N(μ, σ)

3. Normal Distribution
Graphically…

4. Empirical Rule (68-95-99.7 Rule)

5. Example
IQ scores are Normally distributed with a mean of 100 and a std dev of 15.
What % of people have IQ scores…
Between 70 and 130?
Less than 85?
Greater than 145?
Less than 115?
Between 55 and 70?

6. Probability Calculations
If we standardize the distribution by calculating z scores, we create a standard normal distribution where N(0,1) with a mean = 0 and std dev = 1
Standard normal table- a table of areas under the standard normal curve. The table entry for each value z is the area under the curve to the left of z

7. Solving Problems w/ Normal Distributions
State the problem in terms of the observed variable, x. Draw a picture of the distribution and shade the area of interest under the curve.
Standardize and draw a picture. Standardize x to restate the problem in terms of a standard normal variable z. Draw a picture to show the area of interest under the standard normal curve.
Use the table or calculator to find the required area under the curve. Remember the area under the entire curve is 1.
Conclusion. Answer the question in context!

8. Example
IQ scores are Normally distributed with a mean of 100 and a std dev of 15.
What percent of people have IQ scores less than 82? Less than 121?
What percent of people have IQ scores greater than 107?
What percent of people have IQ scores between 88 and 104?

9. Finding a Value (given proportion)
Work backwards to find the value
Draw a normal curve with the area of interest shaded and the mean, standard deviation, and unknown boundary value
Either use the table by finding the area and locating the z value or use the calculator invNorm function to find the z value.
Use the formula for z value to find x
If using invNorm (area to left, μ, σ)
Answer the question in context!

10. Example
A person is considered a genius if they are in the top 2% in terms of IQ. What IQ score does a person need to be considered a genius?

11. Calculator Usage
To calculate the % of observations within a certain interval, use the z-table or the graphing calculator.
2nd Vars (Dist), choose option 2.
Normalcdf (min, max, μ, σ)
To calculate raw data scores from percentiles:
2nd Vars (Dist), choose option 3.
invNorm(%, μ, σ)

12. Assessing Normality Construct a histogram or stemplot
If graph is approximately bell shaped and symmetric about the mean then Normal
Interpret a Normal Probability Plot
If points on plot lie close to a straight line, the data is Normal
Not on the AP exam!
Can you your calculator