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Uncoded Coefficients

You can obtain a regression model using uncoded coefficients by selecting QXL DOE Ribbon --> Analyze Design --> Uncoded Coefficients. When selecting this option, all coding/standardizing instructions are ignored and the X matrix is built using actual factor values.

Example: Temperature (Factor A) has actual values ranging from 20 degrees to 40 degrees. Since Temperature is coded, when using the command “Run Regression” the X matrix is constructed by replacing actual values with coded ones. Specifically, all occurrences of 20 degrees are replaced with --1 and all occurrences of 40 degrees are replaced with +1. If you select “Uncoded Coefficients” then this replacement does not occur. The X matrix is constructed with actual values instead of coded ones.

This option is typically only used after the significance of terms, specifically interactions, has been determined. Using uncoded coefficients is an effective way to get the regression equation without having to code the inputs. However, significance of terms by using p-value should not be accomplished when using uncoded coefficients. When the inputs are not coded, the Variance Inflation Factors are inflated leading to incorrect p-values.