Week 12 Flashcards
1
Q
Regression is?
A
- More Fiddly than other methods
- Has more assumptions
2
Q
Why do Linear Regression
A
- Not looking at differences
- Looking at relationships
- Regression goes further than correlation - Allows us to make predictions
- Produces a model that allows for sophisticated exploration of relationships in variables
3
Q
In Second Year Stats
A
- Looked at relationships - Correlation
- Differences Between Groups and Within-Groups
- Used t-tests and aNOVAs
- Variation in Dependent Variable
4
Q
Correlation
A
Allows us to estimate direction and strength of a linear relationship
5
Q
Why do Linear Regression
A
- How well will a set of variables predict an outcome?
- Which variable in a set of variables is the best predictor of an outcome?
- Does a particular predictor variable predict an outcome if another variable is controlled for?
6
Q
Predictor Variable
A
Same as Independent Variable in Regression
7
Q
Outcome Variable
A
The same as the Dependent Variable in Regression
8
Q
What is a Model?
A
- An approximation to the actual data
- simple summary of data
- Makes data easier to interpret, communicate
- Allows us to predict data
9
Q
What is a Regression Model
A
Mathematically Describes the linear relationship
* Y = Beta X + C
* Y = Predicted valuus of the DV
* Beta = The slope of the line
* X = Scores on the Predictor (IV)
* C = The Intercept
10
Q
The Intercept
A
- Point where the function crosses the y-axis.
- Sometimes Regression model only becomes significant when we remove the intercept, and the regression line reduces to
- Y = b(X) + error
11
Q
Standardized beta (β)
A
- Compares the strength of the effect of each IV to the DV
- The higher the absolute value of the beta coefficient, the stronger the effect
11
Q
How Does Regression Work?
A
- Linear combination of another variable don’t always have to be continuous
- Can have a combination of variables
- Need to find the Line of Best Fit
12
Q
Line of Best Fit
A
- Many lines produced with Regression Formula
- How do we know what line is best?
- Mimimises the difference between observed values and data predicted by the line
- This is called error
- In regression also called residuals
* Y = b(X) + C + error
13
Q
N (Cases): k (Predictors) Ratio
A
- Assumption about sample size
- Need a certain number of participants to trust validity
- Simple Linear Regression Assumption
- Number of Cases multiplied by Predictors
- The more Predictors we have the more cases we need for the study
14
Q
Checking Linearity
A
- checking for Linearity requires scatter plots
- Need Scattepots between each DV & IV
- Looking for Non-Linear evidence
15
Q
Check for Normality
A
- Kolmogorov-Smirnov/Shapiro-Wilks: p > .05
- Skewness & Kurtosis: z score is < ±1.96 then it is normal
- Histogram follows a bell curve.
- Detrended Q-Q Plots: Equal amounts of dots above and below the line.
- Normal QQ Plots: Normal if dots hugging the line.
16
Q
Check for Univariate Outliers
Week 12 Part 2 - 10:00
A
- Identified on Box & Whisker Plots
- Dots indicate outliers
- Asterisk indicates extreme cases
- Number tells you which case is the issue
17
Q
Reason Univariate Outliers are Problematic
A
- Regression Analysis gives formula for a straight line
- A data point that stands outside other data points can change the slope of your straight line
- This makes the line a poor predictor of the value of other data points
17
Q
How to deal with Outliers
A
- Check if Outlier is a data entry error and fix it
- Check if outlier is from different population - Justifies removing their data
- Separate outliers and run different analysis
- Run Analysis with and without outliers and report both models
- Winsorization - Change values so they’re not Outliers anymore
- Use transformations or Bootstrapping
18
Q
Winsorization
A
- Change the score of outlier to value of 5th percentile for minimum values
- Change the score of outlier to value of 95th percentile for maximum values
- Slightly problematic because it changes the data
- But this retains extremeness without removing outlier data
19
Q
Bootstrapping
A
- Uses transformations to deal with outliers
- Creates samples from your sample
- Uses your Mean and Standard Deviation to create another data set
- does this repeatedly
- This creates a large data set where extreme values are more normal
19
Q
Homoscedasticity
A
- Means Scame Scatter or Same Variance
- Residuals are equal for all scores on the Outcome Variable
19
Q
Check for Normality, Linearity and Homoscedasticity
A
- We need the residuals to behave in a certain way
- Residuals are the difference between predicted scores and outcome variable
- SPSS generates a Histogram Q-Q Plots
20
Q
Dealing with Heteroscedasticity
A
- Check graph in SPSS
- Check the data for any patterns
- If dots are scattered randomly then we are all good
22
Q
Null Hypothesis for Regression
A
- Slope of Regression line will be equal to 0
- Beta = 0
23
Q
Alternative Hypothesis
A
- Slope of the Regression line will not be Zero
- Beta Not= 0
24
Q
Running Linear Regression
A
1. Analyse
2. Regression
3. Linear
4. Move your DV into the dependent box.
5. Move your independent variable into the independent box.
6. Ok
25
Q
Linear Regression - R value
A
- Same as Pearson’s Correlation - p value
- Tells us strength and direction of relationship
26
Q
Linear Regression R Square Value
A
- Tells us amount of variance in DV explained by IV
- Proportion of Variance that can be explained by the variable
- 23% of variability in grades explained by attendance in this example
- Known to overestimate the explained variance
27
Q
Linear Regression - R Square Adjusted
A
- R needs to be adjusted to be smaller than R squared
- Corrects bias of overestimated explained variance
- Useful as Goodness of Fit Statistic
29
Q
Regression ANOVA
A
- Uses the df, F value and the p value
- Compares error rate with line of best fit and the error rate of the baseline model of 0
- ANOVA is significant if it is “better” than the baseline
30
Q
Unstandardised Coefficient
A
- The Slope of the Regression Equation
- Amount of change in a Dependent Variable due to a change of an Independent Variable
- This is the Beta coefficient
e.g. each unit of attendance is associated with 1.88 unit of increase in grades
31
Q
Coefficient t-tests
A
- Check if IV is a significant predictor of the DV
- Become more relevant when we start adding more predictors
32
Q
Standardised Coefficients
A
- A measure of the effect size
- Useful for multiple Regression
- Important when we have more than one Predictor
- Predictors often measured in different scales
- e.g, IQ Points, Classes attended, additional study time
33
Q
Dealing with Multiple Predictors
A
- Most commonly found in Research Projects
- Allows us to predict the outome variable from more than one predictor
- Answers how well does combination predictors predict the outcome
Y = b1(X1) + b2(X2) + C + error
39
Q
Mahalanobis Distance
A
- largest value should not be greater than the critical 𝜒2 value for df = k at 𝛼= .001.
- Where k = the number of predictors.
- Use same table as Cook’s Distance
- For simplicity use table below:
42
Q
Check for Multivariate Outliers
A
- Use Residual Statistics Table
- First, we find the critical 𝜒2 for a model with 4 predictors: 𝜒2 = 18.467 - Check the Mahalanobis Distance Table
- Use Mahal. Distance Maximum (13.803 here)
- 13.803 < 18.467 Therefore there are no multivariate outliers.
- Cooks D is < 1 so there are no influential cases
43
Q
Interpreting Multiple Regression
A
- Use the Variables Entered/Removed Table - Tells you how many predictors are in the model (4)
- Then Model Summary Table
- R is not just Person’s R anymore
- It is correlation between actual scores and predictions in the regression equation
- R square = Proportion of variance in DV Accounted for y combined predictors
- Again R square Adjusted is a corrected version of R square that accounts for the positive bias.
44
Q
Interpreting Multiple Regression ANOVA
A
- Now tests the comBination of predictors
- A significant predictor of GHQ
- The table has the df, the F value, and the p-value.
45
Q
Interpreting Multiple Regression Coefficients
A
- Unstandardized coefficient is the slope of the regression
- Shows each unit increase in one of the independent variables is associated with a b unit increase in GHQ
- All other IVs are kept constant
- Beta values = Standardised regression coefficients
- Allow direct comparison of regression coefficients.
- Displayed in units of standard deviation.
46
Q
Interpreting Multiple Regression Standardised Coefficients
A
- t-values and p-values test the significance of the unique contribution of each predictor
- Changes depending on predictors included in the model.
47
Q
Multiple Regression Tolerance & VIF
A
- Tolerance: values < .1 are multicollinear; < .2 warrant closer inspection.
- VIF: values > 10 are clearly multicollinear; > 5 warrant closer inspection.
49
Q
Applied look at Regression Equation
A
- Our general form for the regression is:
Y = b1(X1) + b2(X2) + b3(X3) + C + error - And if we take this equation and substitute in our variables we get:
GHQ = b1(neuroticism) + b2(state-anxiety) + b3(trait-anxiety) + C + error
GHQ = .555(Neuroticism) + .318(state-anxiety) + .471(trait-anxiety) + 13.552 + error
