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Multiple Regression: Multiple R

Multiple regression is the process of generating an equation using a combination of predictor variables to create an expected outcome. The equation appears as follows:

Predicted value of Y = β 1 X 1 + β 2 X 2 + β 3 X 3 + + β i X i .

The equation is true where the β represents the standardized coefficient for each variable X (this is in pairs so that each variable X has a separately estimated coefficient, β). The process involves the use of a combination of predictor variables (X) to estimate an outcome (the Y variable). This equation is considered a standard equation where the expression has been adjusted to remove the scale metric and does not use the raw weights (b). The prediction is based on the value using a z score estimate for each variable in the analysis (X and Y). Each coefficient used (βi) provides the unique contribution for each variable (removing the influence of the other variables in the equation). Unlike a raw equation, the standardized equation has no intercept (constant) because the constant is subtracted (and coefficients standardized) and the values go through the origin.

Defining Multiple R

Multiple R represents essentially the correlation between the predicted value of Y generated in the equation above and the actual value of Y for each unit. The assumption is that the combination of predictors will generate a multiple R or correlation that is larger than any single predictor. If a single predictor can predict or estimate the value of Y as efficiently as a combination of predictors then the justification to employ multiple regression does not exist.

Unlike the typical correlation coefficient that can range from −1.00 to 1.00, multiple R ranges from zero to 1.00. The possibility of negative effects or inverse relationships with predictor variables (X) is considered but introduced by having the regression equation standardized weight (β) represented as negative. So the negative weight has the influence of creating a subtraction for variables that are negatively correlated or inversely related to the outcome variable (Y). The effect of this is to make the multiple R theoretically have no negative values, one factor that distinguishes the multiple R from the normal bivariate correlation (r).

Assumptions of Multiple R

Multiple regression assumes that there exists no multicollinearity among the set of predictor variables (Xi) used in the equation. What happens if multicollinearity exists is that the value for the multiple R is still accurate; however, the contribution of the individual predictors (provided by the standardized regression coefficients, β) becomes inaccurate estimators of the relative contribution of any given predictor variable. The impact, however, on the estimation of multiple R remains unaffected. If the purpose of the use of the equation is the generation of an expected estimate to compare with the actual value, the equation remains useful. The impact of multicollinearity creates conditions where the determination of the relative contribution of any predictor variable (X) remains unclear and the relative size of the coefficients to determine degree of contribution remains unjustified.

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