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Week 11 Correlation & Linear RegressionPowerPoint Presentation

Week 11 Correlation & Linear Regression

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Presentation Transcript

Administrative Tasks

- Turn in your HW
- Sample Research Paper
- Buckle your seatbelts we have a lot to cover

Scatterplots

- Y is the vertical axis
- X is the horizontal axis
- Dots are observations
- The intersection of the IV & DV for each unit of analysis

- What can you infer from about the IV/DV relationship from the scatterplot on the left?
- Presentation can be deceiving

Three factors to consider when analyzing scatterplots

- Directionality
- Do the dots appear to flow in a particular direction?
- Or, does the scatterplot look more like white noise (like a TV when it doesn’t have a signal)?
- The more it looks like white noise the less the two variables are related

- Clustering
- Are the majority of the dots in a small area of the graph?
- How would this impact our confidence in predictions outside of this area?

- Outliers
- Are there cases that differ markedly from the overall pattern of the dots? i.e. not near the cluster or contrary to the directionality
- Which observations are these? How influential are these cases?

Pearsons’s correlation coefficient: quantifying scatter

- To estimate the strength of the relationship between two interval level variables we can calculate Pearson’s correlation coefficient (r)
- Values range from -1 to +1
- -1 = perfectly negative association
- +1 = perfectly positive association
- 0 = no association

- Values range from -1 to +1
- The perks of Pearson:
- Direction
- Magnitude (of predictive power)
- Impervious to the units in which the variables are measured

Scatterplots and r values

Calculating r

- “That’s scarier than anything I saw on Halloween!”
- 1. Subtract each observations x value from x’s mean and multiply it by the difference between its y value and the mean of y
- 2. Do that for each observation and sum them all together
- 3. Divide that sum by n-1 times the s.d. of x & the s.d. of y

The real good news? You don’t have to actually do that

- Excel is your best buddy & will do all the hard work for you
- Not only that, but Excel will also allow you to create a scatterplot and show you a line depicting this relationship
- Let’s check it out:

“Tell me more about this line!”

- Slope:
- Change in Y divided by change in X

- Intercept
- Value of Y when X = O

- Error
- Distance between the line’s Y value and a data point’s Y value

- The line minimizes the sum of all the squared errors
- “I love line!”

Recipe for creating the line

- The line rarely, if ever, passes through every point
- There is an error component
- Thus, the actual values of Y can be explained by the formula:
- Y=α+βX+ε
- α - Alpha – an intercept component to the model that represents the models value for Y when X=0
- β - Beta – a coefficient that loosely denotes the nature of the relationship between Y and X and more specifically denotes the slope of the linear equation that specifies the model
- ε - Epsilon – a term that represents the errors associated with the model

This is ordinary least squares (OLS) or linear regression

- The Goal:
- Minimize the sum of the squared errors

- Consider the impact of outliers
- How many ways can a line be created?

“Not gonna do it. Wouldn’t be prudent.”

- You know the trick:
- It’s not as hard as it looks

- You are really comparing Y’s deviations from it’s mean alongside X’s deviation from it’s mean
- See the formula at the bottom of pg. 331

- Ideally the Xi’s move in sync with the Yi’s divergence from the mean
- This is covariation

The Really Good News? Excel does it all for you!

- Enter the data into Excel
- Click the “Data” tab at the top
- In the Data tab look all the way to the right and click on “Data Analysis”
- In the Analysis Tools menu click on Regression and hit Ok
- Highlight the appropriate columns in the “Input Y Range” & Input X Range” fields
- Check the labels option & hit Ok
- Instant regression results!

What to look for when examining regression output

- Beta coefficient:
- Directionality
- Size of the coefficient
- Standard Error
- Statistical Significance

- Constant
- Far less important than Beta
- When X = O what would we expect Y to be?
- Is X ever O?

- Goodness of fit
- How much of the variation in Y is actually explained by X?
- How “good” does your model “fit” the actual values of Y?
- R-squared (the coefficient of determination) provides an estimate

r Strikes Back!

- Recall that r, Pearson’s correlation coefficient, measures the degree to which two variables co-vary
- With OLS the:
- Constant tells us where the line starts
- Beta tells us how the line slopes
- R-squared tells us the % of the variation in Y our model predicts
- Range 0-1
- O = Predicts none of the variation
- 1 = Predicts all of the variation

- Range 0-1

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