1. 3.1-3.2 Scatterplots and Correlation
AP Statistics
Scatterplots and Correlation

2. Learning Objectives:
Differentiate between an explanatory and response variable
Draw and interpret a scatterplot
Add categorical data into a scatterplot
Calculate and interpret the Correlation between two variables

3. Response Variable-
Measures an outcome of a study 
Explanatory Variable-
Attempts to explain the observed outcomes
We often find explanatory data called independent variable, and response variables called dependent variable.
**This simply tells us the response variable depends on the explanatory variable***

4. State average SAT math score vs. Percent of graduates taking the SAT
Go to page 29 in your textbook (we are just looking at the columns SAT Math and percent taking)
Where does the explanatory data go?
x-axis
Where does the response variable go?
y-axis
Identify each variable (which one is the independent? Dependent?) Also decide if there is a positive or negative association.
Discuss this with your groups and then share your reasoning with the class

5. Independent-
% taking
Dependent-
SAT Math
Remember: you sign up for the exam first, so we first know the percent of kids taking it. Then you take the exam so we find out the average SAT math score for the state last.
It should have a negative association. Think of this:
- If a very small % takes the SAT, they are all most likely taking it to get into college and should be smarter.
- If a large % takes it, the average is lower(think of the ACT at Athens where they pay for everyone to take- some kids who know they aren’t going to college don’t even attempt this test and lower the overall average)

6. Draw a scatter plot (we are just going to input the data from AL to KY to save time)
Step 1: Input % taking into L1
SAT Math into L2
Step 2: 2nd-statplot-1-enter so the cursor is highlighting ON.
Xlist: L1
Ylist:L2
Then hit zoom 9 (this will fit your window to the data you inputted)
Sketch a quick scatter plot on your notes (your axis doesn’t have to start at 0).

7. This is a rough sketch of the scatter plot. Make sure you label it.

8. Give an example with no explanatory-response distinction?
Now take 2 minutes and come up with an example in your groups. -Did anyone say hair color vs. weight? (this is incorrect b/c hair color is categorical and you need 2 variables that are quantitative!!)

9. Examining a scatterplot
We want to look for an overall pattern (linear?) .
We can describe the overall pattern of a scatterplot by the direction, form, and strength!!!
An important kind of deviation is an outlier.

10. Example: Use the scatter plot to answer a-c a) Are there any clusters?
b) Are there any outliers?
c) Is there a clear direction?

12. Positive Association-
As x increases, y increases
Negative Association-
As x increases, y decreases

13. A scatterplot displays the relationship between two quantitative variables.
How can we add categorical variables into a scatterplot?
It would have to be a third variable-then you can use a key!!

14. Show an example:

15. Correlation (r)
The correlation measures the direction and strength of the linear relationship between two quantitative variables. Correlation is usually written as r.

16. 1-The mean and standard deviation of the two variables are denoted as:
2- The correlation between x and y is:
Your calculator does this for you! First-make sure your diagnostics are on.
2nd-catalog (0)-scroll down to diagnostic ON- enter-enter.
(you only have to set your calc to this once, unless you change the batteries)

17. Find the correlation between the two bones in the fossil specimens.
Femur
Humerus
Input Femur into L1
and humerus into L2
Graph it-does it look
postive or negative? Is the correlation strong, moderatley strong, weak?

18. To find the correlation (r):
TI-84: Stat-calc-8 (use 8 not 4)
Xlist:L1
Ylist:L2
FreqList: (leave this blank)
Store RegEQ: Y1 (vars-Yvars-1-1)
Calculate
TI-83: Stat-calc-8 (use 8 not 4)
then type in L1,L2, Y1, enter
r= There is a very strong positive linear relationship between the femur and the humerus.
(make sure you write out the sentence that describes the r value not just r= )

19. When a question asks for the correlation-you have to give the r value and ALSO describe it in a sentence EVERY TIME!!! (that is what they grade you on for the AP exam)
The sentence should include the strength (strong,weak,..), direction (pos. or neg.) and the form (linear)
So if r=0.87 for your test grades versus the hours you studied.
Answer: r=0.87 There is a moderately strong positive linear relationship between hrs. studied and test grades

20. Facts About Correlation
#1-Positive r indicates pos. association
  Negative r indicates negative association
#2- correlation always falls between -1 and 1
(the closer to 1 and -1, the stronger it is)

21. #3-r is standardized, so it does not change with different measurements. (Go back and look at the actual formula for r. It is really just converting x and y’s to z-scores).
#4- correlation measures the strength of only linear relationships b/w 2 variables
#5- Correlation is strongly affected by outliers!
  #6- Correlation is non-directional (flip the x and y doesn’t change it!)
Correlation is not a complete description of two variable data!!!!!