• Skip to primary navigation
  • Skip to main content
  • Skip to primary sidebar
The Analysis Factor

The Analysis Factor

Statistical Consulting, Resources, and Statistics Workshops for Researchers

  • our programs
    • Membership
    • Online Workshops
    • Free Webinars
    • Consulting Services
  • statistical resources
  • blog
  • about
    • Our Team
    • Our Core Values
    • Our Privacy Policy
    • Employment
    • Collaborate with Us
  • contact
  • login

clustered data

Three Designs that Look Like Repeated Measures, But Aren’t

by Karen Grace-Martin  2 Comments

Repeated measures is one of those terms in statistics that sounds like it could apply to many design situations. In fact, it describes only one.

A repeated measures design is one where each subject is measured repeatedly over time, space, or condition on the dependent variable. 

These repeated measurements on the same subject are not independent of each other. They’re clustered. They are more correlated to each other than they are to responses from other subjects. Even if both subjects are in the same condition.  [Read more…] about Three Designs that Look Like Repeated Measures, But Aren’t

Tagged With: autocorrelation, clustered data, communicate results, correlated variable, Repeated Measures

Related Posts

  • Six Differences Between Repeated Measures ANOVA and Linear Mixed Models
  • The Difference Between Clustered, Longitudinal, and Repeated Measures Data
  • Can I Treat 5 Waves of Repeated Measurements as Categorical or Continuous?
  • Linear Mixed Models for Missing Data in Pre-Post Studies

Six Differences Between Repeated Measures ANOVA and Linear Mixed Models

by Karen Grace-Martin  13 Comments

As mixed models are becoming more widespread, there is a lot of confusion about when to use these more flexible but complicated models and when to use the much simpler and easier-to-understand repeated measures ANOVA.

One thing that makes the decision harder is sometimes the results are exactly the same from the two models and sometimes the results are [Read more…] about Six Differences Between Repeated Measures ANOVA and Linear Mixed Models

Tagged With: ANOVA, clustered data, linear mixed model, Missing Data, mixed model, Repeated Measures, repeated measures anova, unbalanced data

Related Posts

  • Linear Mixed Models for Missing Data in Pre-Post Studies
  • Five Advantages of Running Repeated Measures ANOVA as a Mixed Model
  • When Does Repeated Measures ANOVA not work for Repeated Measures Data?
  • Approaches to Repeated Measures Data: Repeated Measures ANOVA, Marginal, and Mixed Models

The Difference Between Clustered, Longitudinal, and Repeated Measures Data

by Karen Grace-Martin  31 Comments

What is the difference between Clustered, Longitudinal, and Repeated Measures Data?  You can use mixed models to analyze all of them. But the issues involved and some of the specifications you choose will differ.

Just recently, I came across a nice discussion about these differences in West, Welch, and Galecki’s (2007) excellent book Linear Mixed Models.

It’s a common question, and there is a lot of overlap in both the study design and in how you will analyze the data from these designs.

West et al give a very nice summary of the three types. Here’s a paraphasing of the differences as they explain them:

In clustered data, the dependent variable is measured once for each subject, but the subjects themselves are somehow grouped (student grouped into classes, for example).  There is no ordering to the subjects within the group, so their responses should be equally correlated.

In repeated measures data, the dependent variable is measured more than once for each subject.  Usually, there is some independent variable (often called a within-subject factor) that changes with each measurement.

And in longitudinal data, the dependent variable is measured at several time points for each subject, often over a relatively long period of time.

A Few Observations

They also make the following good observations:

1. Dropout is usually not a problem in repeated measures studies, in which all data collection occurs in one sitting.  It is a huge issue in longitudinal studies, which usually require multiple contacts with participants for data collection.

2. Longitudinal data can also be clustered.  If you follow those students for two years, you have both clustered and longitudinal data.  You have to deal with both.

3. It can be hard to distinguish between repeated measures and longitudinal data if the repeated measures occur over time.  [My two cents:  A pre/post/followup design is a classic example].

4. From an analysis point of view, it  doesn’t really matter which one you have.  All three are types of hierarchical, nested, or multilevel data. You would analyze them all with some sort of mixed or multilevel analysis.  You may of course have extra issues (like dropout) to deal with in some of these.

My Own Observations

I agree with their observations, and I’d like to add a few from my own experience.

1. Repeated measures don’t have to be repeated over time.  They can be repeated over space (the right knee gets the control operation and the left knee gets the experimental operation). They can also be repeated over condition (each subject gets both the high and low cognitive load condition.  Longitudinal studies are pretty much always over time.

This becomes an issue mainly when you are choosing a covariance structure for the within-subject residuals (as determined by the Repeated statement in SAS’s Proc Mixed or SPSS Mixed).  An auto-regressive structure is often needed when some repeated measurements are closer to each other than others (over either time or space).  This is not an issue with purely clustered data, since there is no order to the observations within a cluster.

2. Time itself is often an important independent variable in longitudinal studies, but in repeated measures studies, it is usually confounded with some independent variable.

When you’re deciding on an analysis, it’s important to think about the role of time.  Time is not important in an experiment, where each measurement is a different condition (with order often randomized).  But it’s very important in a study designed to measure changes in a dependent variable over the course of 3 decades.

3. Time may be measured with some proxy like Age or Order.  But it’s still really about time.

4. A longitudinal study does not have to be over years.  You could be measuring reaction time every second for a minute.  In cases like this, dropout isn’t an issue, although time is an important predictor.

5. Consider whether it makes sense to think about time as continuous or categorical.  If you have only two time points, even if you have numerical measurements for them, there isn’t a point in treating it as continuous.  You need at least three time points to fit a line, but more is always better.

6. Longitudinal datacan be analyzed with many statistical methods, including structural equation modeling and survival analysis.  You only use multilevel modeling if the dependent variable is measured repeatedly and if the point of the model is to see how it changes (or differs).

Naming a data structure, design, or analysis is most helpful if it is so specific that it defines yours exactly.  Your repeated measures analysis may not be like the repeated measures example you’re trying to follow. Rather than trying to name the analysis or the data structure, think about the issues involved in your design, your hypotheses, and your data. Work with them accordingly.

Tagged With: clustered data, Longitudinal Data, mixed model, Repeated Measures

Related Posts

  • Six Differences Between Repeated Measures ANOVA and Linear Mixed Models
  • Three Designs that Look Like Repeated Measures, But Aren’t
  • The Difference Between Crossed and Nested Factors
  • Approaches to Repeated Measures Data: Repeated Measures ANOVA, Marginal, and Mixed Models

Primary Sidebar

This Month’s Statistically Speaking Live Training

  • Member Training: Moderated Mediation, Not Mediated Moderation

Upcoming Workshops

    No Events

Upcoming Free Webinars

TBA

Quick links

Our Programs Statistical Resources Blog/News About Contact Log in

Contact

Upcoming

Free Webinars Membership Trainings Workshops

Privacy Policy

Search

Copyright © 2008–2023 The Analysis Factor, LLC.
All rights reserved.

The Analysis Factor uses cookies to ensure that we give you the best experience of our website. If you continue we assume that you consent to receive cookies on all websites from The Analysis Factor.
Continue Privacy Policy
Privacy & Cookies Policy

Privacy Overview

This website uses cookies to improve your experience while you navigate through the website. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. We also use third-party cookies that help us analyze and understand how you use this website. These cookies will be stored in your browser only with your consent. You also have the option to opt-out of these cookies. But opting out of some of these cookies may affect your browsing experience.
Necessary
Always Enabled
Necessary cookies are absolutely essential for the website to function properly. This category only includes cookies that ensures basic functionalities and security features of the website. These cookies do not store any personal information.
Non-necessary
Any cookies that may not be particularly necessary for the website to function and is used specifically to collect user personal data via analytics, ads, other embedded contents are termed as non-necessary cookies. It is mandatory to procure user consent prior to running these cookies on your website.
SAVE & ACCEPT