1. AP Stat Day 10 68 Days until AP Exam
Describing Data with Numbers
Box and Whisker Plots
Variance and Standard Deviation

2. Today’s Objectives
Content: I can display data in a box and whisker plot and calculate outliers and standard deviation.
Language: I can work cooperatively to create a box and whisker plot using class data.

3. Calculating a Mean… In statistics, the mean has a special symbol:
which is said “x-bar”
The formula for calculating the mean is:
Or, more simply:

4. Calculating a Median Order the numbers from smallest to largest.
If the number of observations, n, is odd then the median is the middle number.
If the number of observations, n, is even, then the median is the mean of the two middle numbers.

5. Comparing the mean and the median
The median is a resistant measure of center.
This means that outliers and skewness do not effect the location of the median.
The mean is a non-resistant measure of spread.
This means that outliers and skewness strongly effect the mean.

6. Skewed Distributions
In a perfectly symmetric distribution, the mean median and mode lie in the same place.
In a right (positive) or left (negative) skewed distribution, the mean is pulled towards the tail. The mode remains at the highest point of the distribution.

7. Quartiles
The median is the center point for a set of data. There is 50% of the data on each side of the median.
A quartile contains 25% of a set of data.
The first quartile, Q1, is the median of the bottom half of the data.
The third quartile, Q3, is the median of the upper half of the data.

8. EXAMPLE Find the quartiles of the following data. 100 92 45 88 91 3672 63 56 83 97 77 65 81 51 78 86 21
Find the quartiles of the following data.
Begin by ordering the numbers.
Find the median.
Find the median of the lower numbers.
Find the median of the upper numbers.
If there is an odd number of observations, leave out the median.
If there is an even number of observations, include all numbers in the list (the median is not in the list.

9. The IQR and Outliers The IQR is short for “Interquartile Range”
To calculate IQR, IQR = Q3 - Q1
Outliers are calculated using the IQR.
The rule for outliers is that if a value is outside 1.5(IQR) then it is an outlier.
So, if a value is more than Q (IQR) or less than Q1 – 1.5(IQR) then it is an outlier.

10. Example
100 92 45 88 91 36 72 63 56 83 97 77 65 81 51 78 86 21
Using the data from the last example, determine whether the data contains any outliers.

11. The 5 Number Summary
The 5 number summary is the list of values used to create a box and whisker plot.
Minimum, Q1, Median, Q3, Maximum
We have already gathered all of this data, we just need to list it.
Ready to use your calculator?

12. Drawing a Box and Whisker Plot
Once you have your 5 number summary, drawing a box and whisker plot is easy.
Important features: a number line, an x-axis title.
The modified box plot

13. Activity
On pg. 21, use the information that we gathered on class heights (back of pg 6) to make a box and whisker plot by hand.
You may use the calculator for the 5 number summary, but you must make the box and whisker plot by hand. You must check to see if there are any outliers.

14. Box Plots in the Calculator
We will use the height data already stored in the calculator from your activity.

15. Variance
Variance is a number that shows the variation around the mean.
The formula for calculating variance is:
or, more simply:

16. Example
Find the variance of the following list of numbers: 5, 7, 3, 12, 4
Find the mean
Find the difference of the mean and each observation
Square the differences
Sum the difference
Divide by n - 1

17. Standard Deviation
The standard deviation is the square root of the variance.
Find the standard deviation of the previous list of numbers.

18. About Standard Deviation
s measures spread about the mean
s = 0 only when there is no spread (i.e. all observations have the same value)
s, like x-bar, is not resistant! So, outliers or skewness makes s very large.

19. X-bar and s or 5 number summary?
If your data is skewed or has strong outliers- use the 5 number summary!
If your data is reasonably symmetric and free of outliers- use x-bar and s!

20. Summary (p 22) What numbers make up the 5 number summary?
How does a modified box plot differ from a traditional box and whisker plot?
If you know a variance, how can you find a standard deviation?

21. Prep Questions (p 23)
Have you ever heard of a bell curve? What does it look like (words)? Sketch one.
What does probability mean?
What is density?