1. Chapter 8 (3-4), 9 More about Correlation

2. Today’s Lecture l SD Line l Calculating r l correlation vs causation

3. The SD Line l the line the points cluster around l passes through the point of averages: (AVGx, AVG Y ) l Has slope :

6. Calculating r (call variables “X” and “Y”) l Step 1: Calculate AVGx and AVGy l Step 2: Calculate SDx and SDy l Step 3: Standardize each variable l Step 4: Find average of products of z- scores (standard scores)

10. NOTE: The Correlation Coefficient is unaffected if the units of measurement are changed Example: Correlation between height and weight remains the same whether height is measured in inches, cm., feet, etc.

13. Important Note: Correlation DOES NOT Imply Causation l strong association between 2 variables is not enough to justify conclusions about cause and effect

14. Examples Strong association between: l number of firefighters and amount of damage –Does sending more firefighters cause more damage? l shoe size and score on a reading comprehension exam for elementary school children –What’s the explanation? l SAT and GPA scores –What’s the explanation?

15. Important Note: Correlation DOES NOT Imply Causation l strong association between 2 variables is not enough to justify conclusions about cause and effect l best way to get evidence that X causes Y is through a controlled experiment