A polynomial term–a quadratic (squared) or cubic (cubed) term turns a linear regression model into a curve.  But because it is X that is squared or cubed, not the Beta coefficient, it still qualifies as a linear model.  This makes it a nice, straightforward way to model curves without having to model complicated non-linear models. […]

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 […]

I just heard recently about PSPP, which is a free, open source version of SPSS.

If you’ve ever been puzzled by odds ratios in a logistic regression that seem backward, stop banging your head on the desk. Odds are (pun intended) you ran your analysis in SAS Proc Logistic. Proc logistic has a strange (I couldn’t say odd again) little default.  If your dependent variable Y is coded 0 and […]

How do you choose which statistical software to use and how many should you learn?

ANOVA and Linear Regression are not only related, they're the same thing. Not a quarter and a nickel--different sides of the same coin. This article shows why.

there is often good justification for using a variety of statistical approaches. This article outlines 5 issues to consider in choosing an analysis where you have options. These are all issues I take into account with my statistical consulting clients.

In an ANOVA or regression model, should you drop interaction terms if they're not significant? As with everything in statistics, it depends.

A Linear Regression Model with an interaction between two predictors changes the meaning of the coefficients for the lower order terms--the predictors that are involved in the interaction. They need to be interpreted differently.

I just came across this great article by Frank Harrell:  Problems Caused by Categorizing Continuous Variables It’s from the Vanderbilt University biostatistics department, so the examples are all medical, but the points hold for any field. It goes right along with my recent post, Continuous and Categorical Variables: The Trouble with Median Splits.