Karen Grace-Martin

When the Hessian Matrix Goes Wacky

December 20th, 2023 by

If you have run mixed models much at all, you have undoubtedly been haunted by some version of this very obtuse warning: “The mixed model Hessian (or G or D) Matrix is not positive definite. Convergence has stopped.”

Or “The Model has not Converged. Parameter Estimates from the last iteration are displayed.”

What on earth does that mean?

Let’s start with some background. If you’ve never taken matrix algebra, (more…)


The Wide and Long Data Format for Repeated Measures Data

December 2nd, 2023 by

One issue in data analysis that feels like it should be obvious, but often isn’t, is setting up your data.

The kinds of issues involved include:

  • What is a variable?stage 1
  • What is a unit of observation?
  • Which data should go in each row of the data matrix?

Answering these practical questions is one of those skills that comes with experience, especially in complicated data sets.

Even so, it’s extremely important. If the data isn’t set up right, the software won’t be able to run any of your analyses.

And in many data situations, you will need to set up the data different ways for different parts of the analyses. (more…)


The Effect Size: The Most Difficult Step in Calculating Sample Size Estimates

June 12th, 2023 by

One of the most difficult steps in calculating sample size estimates is determining the smallest scientifically meaningful effect size.

Here’s the logic:

The power of every significance test is based on four things: the alpha level, the size of the effect, the amount of variation in the data, and the sample size.

You will measure the effect size in question differently, depending on which statistical test you’re performing. It could be a mean difference, a difference in proportions, a correlation, regression slope, odds ratio, etc.

When you’re planning a study and estimating the sample size needed for (more…)


Can Likert Scale Data ever be Continuous?

January 19th, 2023 by

A very common question is whether it is legitimate to use Likert scale data in parametric statistical procedures that require interval data, such as Linear Regression, ANOVA, and Factor Analysis.

A typical Likert scale item has 5 to 11 points that indicate the degree of something. For example, it could measure agreement with a statement, such as 1=Strongly Disagree to 5=Strongly Agree. It can be a 1 to 5 scale, 0 to 10, etc. (more…)


Getting Started with SPSS Syntax

December 22nd, 2022 by

spss-logoYou may have heard that using SPSS syntax is more efficient, gives you more control, and ultimately saves you time and frustration.  It’s all true.

….And yet you probably use SPSS because you don’t want to code.  You like the menus.

I get it.

I like the menus, too, and I use them all the time.

But I use syntax just as often.

At some point, if you want to do serious data analysis, you have to start using syntax.  (more…)


Three SPSS Shortcuts that Make Life Easier

October 24th, 2022 by

Okay, maybe these SPSS shortcuts won’t make your whole life easier, but it will help your work life, at least the SPSS part of it.

When I consult with researchers, a common part of that is going through their analysis together.  Sometimes I notice that they’re using some shortcut in SPSS that I had not known about.

Or sometimes they could be saving themselves some headaches.

So I thought I’d share three buttons you may not have noticed before that will make your data analysis more efficient.

(more…)