dummy coding

Dummy Coding in SPSS GLM–More on Fixed Factors, Covariates, and Reference Groups, Part 2

March 31st, 2009 by

Part 1 outlined one issue in deciding whether to put a categorical predictor variable into Fixed Factors or Covariates in SPSS GLM.  That issue dealt with how SPSS automatically creates dummy variables from any variable in Fixed Factors.

There is another key default to keep in mind. SPSS GLM will automatically create interactions between any and all variables you specify as Fixed Factors.

If you put 5 variables in Fixed Factors, you’ll get a lot of interactions. SPSS will automatically create all 2-way, 3-way, 4-way, and even a 5-way interaction among those 5 variables. (more…)

Why ANOVA and Linear Regression are the Same Analysis

March 11th, 2009 by

Stage 2If your graduate statistical training was anything like mine, you learned ANOVA in one class and Linear Regression in another.  My professors would often say things like “ANOVA is just a special case of Regression,” but give vague answers when pressed.

It was not until I started consulting that I realized how closely related ANOVA and regression are.  They’re not only related, they’re the same thing.  Not a quarter and a nickel–different sides of the same coin.

So here is a very simple example that shows why.  When someone showed me this, a light bulb went on, even though I already knew both ANOVA and multiple linear (more…)

Interpreting Lower Order Coefficients When the Model Contains an Interaction

February 23rd, 2009 by

A Linear Regression Model with an interaction between two predictors (X1 and X2) has the form: 

Y = B0 + B1X1 + B2X2 + B3X1*X2.

It doesn’t really matter if X1 and X2 are categorical or continuous, but let’s assume they are continuous for simplicity.

One important concept is that B1 and B2 are not main effects, the way they would be if (more…)

Concepts in Linear Regression you need to know before learning Multilevel Models

January 30th, 2009 by

It seems very many researchers are needing to learn multilevel and mixed models, and I have to say, it’s not so easy on your own.

I too went to graduate school before it was taught in classes–we did learn mixed models as in Split Plot designs, but things have progressed a bit since then.  So I too have had to learn them without benefit of a class, or teacher.  So I feel your pain.  But I’ve struggled through and learned a (more…)

SPSS GLM: Choosing Fixed Factors and Covariates

December 30th, 2008 by

The beauty of the Univariate GLM procedure in SPSS is that it is so flexible.  You can use it to analyze regressions, ANOVAs, ANCOVAs with all sorts of interactions, dummy coding, etc.

The down side of this flexibility is it is often confusing what to put where and what it all means.

So here’s a quick breakdown.

The dependent variable I hope is pretty straightforward.  Put in your continuous dependent variable.

Fixed Factors are categorical independent variables.  It does not matter if the variable is (more…)