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Solving for and b

- The weight for predictor xj will be a function of:
- The correlation xj and y.
- The extent to which xj’s relationship with y is redundant with other predictors relationships with y (collinearity).
- The correlations between y and all other predictors.
- The correlations between xj and all other predictors

Solving for and b: the two variable case

- 1 = slope for X1 controlling for the other independent variable X2
- 2 is computed the same way. Swap X1s, X2s
- Compare to bivariate slope:
- What happens to b1 if X1 and X2 are totally uncorrelated?

Solving for and b: the two variable case

- Solving for and b is relatively simple with two variables but becomes increasingly complex with more variables and requires differential calculus to derive formulas. Matrix algebra can be used to simplify the process.

Matrix Equations

- R2 = S(ryjbj)
- where each ryj = correlation between the DV and the jthIV
- each bi = standardized regression coefficient

- R2 = RyjBj
- where Ryj = row matrix of correlations between the DV and k IVs.
- Bj = column matrix of standardized regression coefficients for the IVs.

- Bj = Rjj-1Rjy
- In other words, the matrix of standardized regression coefficients is simply the correlation matrix between the DV and IVs divided by the matrix of correlations among the IVs.

Tests of Regression Coefficients

The Xj predictor is not related to Y when the other predictors are held constant.

Null Hypothesis:

Further Interpretation of Regression Coefficients

- Regression coefficients in multiple regression (unstandardized and standardized) are considered partial regression coefficients because each coefficient is calculated after controlling for the other predictors in the model.
- Tests of regression coefficients represent a test of the unique contribution of that variable in predicting y over and above all other predictor variables in the model.

Assumptions

- Predictors are linearly related to criterion.
- Normality of errors -- residuals are normally distributed around zero
- Multivariate normal distribution -- multivariate extension of bivariate normality—homoscedasticity.
Regression diagnostics check on these assumptions

Regression Diagnostics

- Detecting multivariate outliers
- Distance, leverage, and influence

- Evaluating Collinearity

Regression Diagnostics

- Methods for identifying problems in your multiple regression analysis -- a good idea for any multiple regression analysis
- Can help identify
- violation of assumptions
- outliers and overly influential cases—cases you might want to delete or transform
- important variables you’ve omitted from the analysis

Three Classes of MR Diagnostic Statistics

1. Distance -- detects outliers in the dependent variable and assumption violations -- primary measure is the residual (Y-Y^) or standardized residual (i.e., put in terms of z scores) or studentized residual (i.e., put in terms of t-scores)

2. Leverage -- identifies potential outliers in the independent variables -- primary measure is the leverage statistic or “hat” diagnostic

Three Classes of MR Diagnostic Statistics (cont.)

3. Influence -- combines distance and leverage to identify unusually influential observations (i.e., observations or cases that have a big influence on the MR equation) -- the measure we will use is Cook’s D

Distance

- Analyze residuals
- Pay attention to standardized or studentized residuals > 2.5; shouldn’t be more than 5% of cases
- Tells you which cases are not predicted well by regression analysis -- you can learn from this in itself
- Necessary to test MR assumptions
- homoscedasticity
- normality of errors

Distance

- Unstandardized Residuals
- The difference between an observed value and the value predicted by the model. The mean is 0.

- Standardized Residuals
- The residual divided by an estimate of its standard error. Standardized residuals have a mean of 0 and a standard deviation of 1.

- Studentized Residuals
- The residual divided by an estimate of its standard deviation that varies from case to case, depending on the leverage of each case’s predictor values in determining model fit. They have a mean of 0 and a standard deviation slightly larger than 1.

Distance

- Deleted Residuals
- The residual for a case that is excluded from the calculation of the regression coefficients. It is the difference between the value of the dependent variable and the adjusted predicted value.

- Studentized Deleted Residuals
- It is a studentized residual with the effect of the observation deleted from the standard error. The residual can be large due to distance, leverage, or influence. The mean is 0 and the variance is slightly greater than 1.

Distance-example

- Open mregression1/example2c.sav
- Regress problems on peak, week, and index
- Under statistics select estimates, covariance matrix, and model fit.
- Save predicted values unstandardized and save all residuals (unstandardized, standardized, Studentized, deleted, and Studentized deleted)
- Okay

Distance-example output

- Interpret b’s and betas. Compare betas with correlations.
- Zero order correlations
- Validity coefficients

- Why is the standard error of estimate different from the standard deviation of unstandardized residuals?
- Note casewise diagnostics compared to saved values.

Leverage (“hat” diagnostic; hat diag)

- Tells you outliers on X variables
- Note that this can detect so-called multivariate outliers, that is, cases that are not outliers on any one X variable but are outliers on combinations of X variables. Example: Someone who is 60 inches tall and weighs 190 pounds.
- Guideline: Pay attention to cases with centered leverage that stands out or is greater than 2*k/n for large samples or 3*k/n for small samples (.04 in this case). (SPSS prints out the centered leverage for each case and the average centered leverage across cases)

Leverage (“hat” diagnostic; hat diag)

- Possible values on leverage range from a low of 1/N to a high of 1.0.
- The mean of the leverage values is k/N
- Rerun the regression but save leverage values (only)
- Examine leverage values > .04.

Influence Statistics

- Cook’s D
- A measure of how much MSResidual would change if a particular case were excluded from the calculations of the regression coefficients. A large Cook’s D indicates that excluding a case from computation of the regression statistics changes the coefficients substantially.

- Dfbeta(s)
- The difference in beta value is the change in the regression coefficient that results from the exclusion of a particular case. A value is computed for each term in the model, including the constant.

- Standardized Dfbeta(s)
- Standardized difference in the beta value.The change in the regression coefficient that results from the exclusion of a particular case. You may want to examine cases with absolute values greater than 2/sqrt(N), where N is the number of cases.

Influence

- There is no general rule for what constitutes a large value of Cook’s D
- Cook’s D > 1 is unusual
- Look for cases that have a large Cook’s D relative to other cases
- Rerun analyses and save Cook’s, dfBetas, and standardized dfBetas (only)

Suggestions for Handling Outliers

- Check for recording errors.
- Determine if they are legitimate cases for the population you intended to sample.
- Transform variables. You sacrifice interpretation here and it doesn’t help much for floor or ceiling effects.
- Trimming (delete extreme cases)
- Winsorizing (assign extreme cases the highest value considered reasonable
- (e.g., if someone reports 99 drink/week, and the next heaviest drinker is 50, change the 99 to 50.)

- Run analyses with and without outliers. If conclusions don’t change, leave them in. If they do change and you take them out, provide justification for removal and note how they affected the results.

Collinearity Among the Predictors

- Identifying the Source(s) of Collinearity
- Tolerance
- Variance Inflation Factor
- Condition Indices and Variance Proportions

- Handling Collinearity

Collinearity

- Collinearity
- We want the predictors to be highly correlated with the dependent variable.
- We do not want the predictors to be highly correlated with each other.
- Collinearity occurs when a predictor is “too” highly correlated with one or more of the other predictors.

- Impact of Collinearity
- The regression coefficients are very sensitive to minor changes in the data.
- The regression coefficients have large standard errors, which lead to low power for the predictors.
- In the extreme case, singularity, you cannot calculate the regression equation.

Tolerance

- Tolerance tells us:
- The amount of overlap between the predictor and all other remaining predictors.
- The degree of instability in the regression coefficients.

- Tolerance values less than 0.10 are often considered to be an indication of collinearity.

Variance Inflation Factor

- The VIF tells us:
- The degree to which the standard error of the predictor is increased due to the predictor’s correlation with the other predictors in the model.

- VIF values greater than 10 (or, Tolerance values less than 0.10) are often considered to be an indication of collinearity.

Condition Indices and Variance Proportions

- Taken together, they provide information about…
- whether collinearity is a concern
- if collinearity is a concern, which predictors are “too” highly correlated
- ‘Weak Dependencies’ have condition indices around 5-10 and two or more variance proportions greater than 0.50.
- ‘Strong Dependencies’ have condition indices around 30 or higher and two or more variance proportions greater than 0.50.

Collinearity diagnostics

- Eigen value -- the amount of total variation that can be explained by one dimension among the variables -- when several are close to 0, this indicates high multicollinearity. Ideal situation: all 1’s.
- Condition index --square root of the ratio of each eigen value to each successive eigen value; > 15 indicates possible problem and > 30 indicates serious problem with multicollinearity

Collinearity diagnostics (cont.)

- Variance proportions -- proportion of each variable explained by a given dimension -- multicollinearity can be a problem when a dimension explains a high proportion of variance in more than one variable. The proportions of variance for each variable shows the “damage” multicollinearity does to estimation of the regression coefficient for each.

Example

- Regress problems on week, peak, and index.
- Under statistics select collinearity diagnostics
- Okay
- Examine tolerance, VIF, and collinearity diagnostics

Collinearity in practice

- When is collinearity a problem?
- When you have predictors that are VERY highly correlated (>.7).

- Mention centering, product terms, and interactions interpretation of coefficients.

Handling Collinearity

- Combine the information contained in your predictors, linear combinations (mean of z-scored predictors), factor analysis, SEM.
- Delete some of the predictors that are too highly correlated.
- Collect additional data…in the hope that additional data will reduce the collinearity.

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