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Chapter 8 (3-4), 9

Chapter 8 (3-4), 9. More about Correlation. Today’s Lecture. SD Line Calculating r correlation vs causation. The SD Line. the line the points cluster around passes through the point of averages: (AVGx , AVG Y ) Has slope : . Calculating r (call variables “X” and “Y”).

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Chapter 8 (3-4), 9

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

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

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

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

  5. 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.

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

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

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

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