An Overview of Nonequivelent Group Study Designs in Wound Care
A Quick Intro to Nonequivelent Group Designs
Much of wound-care research is not randomized control studies, but rather quasi-experimental studies. The most commonly used quasi-experimental design is the nonequivalent group design (NEGD)1.
According to Reichardt, “in the nonequivalent group design, groups of participants are non-randomly assigned to receive either a treatment or a comparison condition … the nonequivalent group design is akin to a between-groups randomized experiment in that both designs draw comparisons between participants who receive different treatment conditions”2.
The notation for the Study Design Diagrams is straightforward:
NR = Not Randomized
X = Treatment | X∀ = A specific treatment
O = Observation | On = Observation Number
I refer to control groups below in the description of NEGD’s. However, pure controls don’t really (at least purposefully!) exist in wound care. If a patient has a wound they are generally being treated using Standard-of-Care (SOC). So really SOC is the control. However, this takes us down a rabbit hole as there manifold SOC’s in the real world. Although there are national guidelines and the like, a SOC in a wound care center may differ significantly from a SOC in a post-acute mobile practice, or a home-care practice. Even SOC may differ between providers in the same practice. However, regardless of the noise in definition, control can be read to mean SOC.
In wound-care, these, in my opinion, are the most prevalent NEGD’s:
Posttest-only Nonequivelent Group Design
This is the simplest NEGD. This design has no pretest (or observation before treatment for neither the control nor the control group). The treatment is administered the treatment group and the only measurements collected are at the end of the study. The control and treatment groups are only compared on post endpoint differences. This design lacks a pretest, so we are unable to assess the magnitude of change or potential bias at pretest.
| NR | X | O1 | |
| NR | O1 |
The Pretest-Posttest Nonequivelent Group Design
This is the most common NEGD. A Pre-test measurement is taken in both groups, a treatment administered in the treatment group, and endpoints assessed at the end of the study. This setup allows allows one to asses the pre and post group differences, and allows for the assessment of change intra and inter group.
| NR | O1 | X | O2 |
| NR | O1 | O2 |
Another variation of this design is to add pre observations, so that the diagram looks something like this:
| NR | O1 | O2 | X | O3 |
| NR | O1 | O2 | O3 |
This extra observation allows for a quasi run-in period that allows the researcher to assess for change between pre-test observations (was one group already healing/worsening before O2?). One could also add post-test observations to assess relapse/recurrence. That could look something like this:
| NR | O1 | X | O2 | O3 |
| NR | O1 | O2 | O3 |
The Pretest-Posttest Alternative Treatments Nonequivelent Group Design
A variation on the the Pretest-Postest NEGD to switch out the control for an alternative treatment. Hence there are two treatment groups. The same addition of pre-test and post-test observations as in Pretest-Postest NEGD.
| NR | O1 | XA | O2 |
| NR | O1 | XB | O2 |
Multiple Treatments and Controls with Pretest
Another variation of the Pretest-Postest NEGD is to simply add another treatment group, which would look like this. The same addition of pre-test and post-test observations as in Pretest-Postest NEGD.
| NR | O1 | XA | O2 |
| NR | O1 | XB | O2 |
| NR | O1 | O2 |
Notwithstanding concerns around bias and internal validity, NEGD are necessary in wound care and form the underpinning of the notion of Real World Evidence (RWE). Understanding their structure, and the appropriate tools for their analysis (more on that to come) is important as we advance wound-care research.
Footnotes
In wound care, it is more commonly called an observational study design, the nomenclature made popular by Cochran and Rosenbaum. However, my preference is for the nomenclature from Campbell, which is more contemporary, and I believe captures the idea of what is actually occurring more accurately (we are, more often than not, not really just observing, are we?).↩︎
Reichardt, C. S., & Little, T. D. (2019). Quasi-experimentation: A guide to design and analysis. The Guilford Press. , p. 140↩︎