General Linear Model

Member Training: Statistical Contrasts

March 31st, 2021 by

Statistical contrasts are a tool for testing specific hypotheses and model effects, particularly comparing specific group means.


Member Training: Confusing Statistical Terms

February 28th, 2020 by

Learning statistics is difficult enough; throw in some especially confusing terminology and it can feel impossible! There are many ways that statistical language can be confusing.

Some terms mean one thing in the English language, but have another (usually more specific) meaning in statistics.  (more…)

The Four Stages of Statistical Skill

September 21st, 2018 by

At The Analysis Factor, we are on a mission to help researchers improve their statistical skills so they can do amazing research.

We all tend to think of “Statistical Analysis” as one big skill, but it’s not.

Over the years of training, coaching, and mentoring data analysts at all stages, I’ve realized there are four fundamental stages of statistical skill:

Stage 1: The Fundamentals



Stage 2: Linear Models




Stage 3: Extensions of Linear Models





Stage 4: Advanced Models




There is also a stage beyond these where the mathematical statisticians dwell. But that stage is required for such a tiny fraction of data analysis projects, we’re going to ignore that one for now.

If you try to master the skill of “statistical analysis” as a whole, it’s going to be overwhelming.

And honestly, you’ll never finish. It’s too big of a field.

But if you can work through these stages, you’ll find you can learn and do just about any statistical analysis you need to. (more…)

When Assumptions of ANCOVA are Irrelevant

October 15th, 2012 by

Every once in a while, I work with a client who is stuck between a particular statistical rock and hard place. It happens when they’re trying to run an analysis of covariance (ANCOVA) model because they have a categorical independent variable and a continuous covariate.

Stage 2

The problem arises when a coauthor, committee member, or reviewer insists that ANCOVA is inappropriate in this situation because one of the following ANCOVA assumptions are not met:

1. The independent variable and the covariate are independent of each other.

2. There is no interaction between independent variable and the covariate.

If you look them up in any design of experiments textbook, which is usually where you’ll find information about ANOVA and ANCOVA, you will indeed find these assumptions.  So the critic has nice references.

However, this is a case where it’s important to stop and think about whether the assumptions apply to your situation, and how dealing with the assumption will affect the analysis and the conclusions you can draw. (more…)

Confusing Statistical Term #7: GLM

August 9th, 2012 by

Like some of the other terms in our list–level and  beta–GLM has two different meanings.

It’s a little different than the others, though, because it’s an abbreviation for two different terms:

General Linear Model and Generalized Linear Model.

It’s extra confusing because their names are so similar on top of having the same abbreviation.

And, oh yeah, Generalized Linear Models are an extension of General Linear Models.

And neither should be confused with Generalized Linear Mixed Models, abbreviated GLMM.

Naturally. (more…)

Checking the Normality Assumption for an ANOVA Model

May 21st, 2012 by

I am reviewing your notes from your workshop on assumptions.  You have made it very clear how to analyze normality for regressions, but I could not find how to determine normality for ANOVAs.  Do I check for normality for each independent variable separately?  Where do I get the residuals?  What plots do I run?  Thank you!

I received this great question this morning from a past participant in my Assumptions of Linear Models workshop.

It’s one of those quick questions without a quick answer. Or rather, without a quick and useful answer.  The quick answer is:

Do it exactly the same way.  All of it.

The longer, useful answer is this: (more…)