What Is an Effect Size in EMDR Research?
An effect size is a statistical measurement that shows how large a difference or change is in a research study.
In Eye Movement Desensitization and Reprocessing (EMDR) research, effect sizes help researchers understand the magnitude of improvements in symptoms and how EMDR compares with other treatments or control groups.
Effect sizes are important because they provide information beyond whether a study found a statistically significant result. They help researchers evaluate how substantial the observed differences are and compare findings across multiple studies.
What Does an Effect Size Mean in EMDR Research?
An effect size describes the magnitude of a treatment-related difference or change.
For example, researchers studying EMDR for Post-Traumatic Stress Disorder (PTSD) may measure participants' symptoms before and after treatment. They may also compare participants receiving EMDR with a separate group receiving another treatment or usual care.
An effect size helps quantify the difference observed in these measurements.
A larger effect size generally indicates a larger difference on the outcome being studied, while a smaller effect size indicates a smaller difference.
However, an effect size does not automatically tell researchers whether a treatment produced meaningful improvement in everyday life. Its interpretation depends on what was measured, the comparison being made, and the quality of the study.
How Are Effect Sizes Calculated in EMDR Studies?
Researchers calculate effect sizes using statistical methods suited to the type of data collected.
One common method is the standardized mean difference, which expresses the difference between two groups relative to the amount of variation in their scores.
For example, imagine a study in which researchers measure PTSD symptoms after treatment:
Participants receiving EMDR have an average symptom score of 30.
Participants in a comparison group have an average symptom score of 40.
Lower scores indicate fewer PTSD symptoms.
The difference between the two groups is 10 points.
Researchers can use the variation in participants' scores to express that difference as a standardized effect size.
If the pooled standard deviation—a measure of how widely scores vary—is 20 points, the standardized difference is 0.50.
This means the groups differ by half a standard deviation.
Standardizing the difference makes it easier to compare results from studies that use different symptom measurement scales.
The Cochrane Handbook explains how standardized mean differences allow researchers to combine findings from studies measuring the same outcome with different instruments.
What Is Considered a Small, Medium, or Large Effect Size?
Researchers sometimes use general guidelines to describe the magnitude of standardized effect sizes.
A commonly used approach comes from statistician Jacob Cohen.
| Effect size | Common interpretation |
|---|---|
| 0.20 | Small effect |
| 0.50 | Medium effect |
| 0.80 | Large effect |
These values are approximate guidelines rather than fixed rules.
For example, a standardized effect size of 0.80 represents a larger measured difference than an effect size of 0.20. However, that does not automatically mean the larger effect is more important for every participant or clinical situation.
The Cochrane Handbook notes that the practical importance of an effect depends on the outcome being measured and its clinical context.
Why Do Effect Sizes Matter in EMDR Research?
Effect sizes help researchers understand the strength of observed treatment differences.
A study might report that participants receiving EMDR experienced statistically significant reductions in PTSD symptoms. However, statistical significance alone does not reveal how large those reductions were.
Effect sizes provide additional information about the magnitude of the findings.
They are particularly useful for three purposes.
1. Measuring the Magnitude of Symptom Improvement
Researchers use effect sizes to describe changes in outcomes such as PTSD symptoms, anxiety, depression, and trauma-related distress.
For example, two studies might both find statistically significant improvements following EMDR treatment, but one might report a larger effect size.
This suggests that the measured change or difference was larger relative to the variation in scores.
However, the interpretation also depends on whether the effect size describes improvement within a treatment group or a difference between treatment groups.
These are not the same comparison.
2. Comparing EMDR With Other Treatments
Effect sizes allow researchers to compare treatment outcomes across groups.
For example, a study may compare EMDR with trauma-focused Cognitive Behavioral Therapy (CBT), a psychological treatment that helps people address distressing thoughts and responses associated with traumatic experiences.
Researchers can calculate an effect size representing the difference in PTSD symptom scores between the groups.
An effect size close to zero generally indicates little measured difference between the treatments. A larger effect size indicates a greater difference, with its direction showing which treatment was favored.
Importantly, a small difference does not necessarily establish that two treatments are equally effective.
3. Combining Results From Multiple Studies
Effect sizes are especially useful in meta-analyses.
A meta-analysis is a research method that statistically combines findings from multiple studies examining related questions.
Different EMDR studies may use different PTSD assessment tools, making direct comparisons of raw symptom scores difficult.
Researchers can use standardized effect sizes to place these results on a comparable scale.
This allows them to estimate an overall treatment effect while examining how much results vary between studies.
What Have EMDR Studies Reported About Effect Sizes?
Several systematic reviews and meta-analyses have reported effect sizes for EMDR treatment.
A 2014 meta-analysis by Chen and colleagues examined 26 randomized controlled trials of EMDR for PTSD.
The researchers reported standardized effect sizes for several outcomes:
| Outcome measured | Reported effect size |
|---|---|
| PTSD symptoms | -0.662 |
| Depression symptoms | -0.643 |
| Anxiety symptoms | -0.640 |
| Subjective distress | -0.956 |
The negative values indicated reductions in symptom scores under the study's calculation method.
The analysis found statistically significant improvements across the measured outcomes, although effect sizes alone cannot establish the quality of the underlying studies.
Source: Chen et al. (2014), meta-analysis of randomized controlled trials.
A separate 2020 systematic review and meta-analysis by Cuijpers and colleagues examined 76 randomized trials involving EMDR across mental health conditions.
The researchers reported an effect size of 0.93 when comparing EMDR with control conditions and 0.36 when comparing EMDR with other psychological therapies.
However, the authors also identified substantial variation between studies, possible publication bias, and methodological limitations. The advantage over other therapies was not found when the analysis was restricted to studies with a low risk of bias.
Source: Cuijpers et al. (2020), systematic review and meta-analysis.
These examples demonstrate why effect sizes need to be interpreted alongside study design, comparison conditions, and research quality.
Is a Large Effect Size the Same as a Statistically Significant Result?
No. Effect size and statistical significance describe different aspects of a research finding.
Effect size describes how large an observed difference or association is.
Statistical significance helps researchers assess whether a result is compatible with a hypothesis of no effect, taking sampling uncertainty into account.
A study may find a small effect that is statistically significant, particularly when the sample size is large.
Another study may report a relatively large effect that is not statistically significant because its sample is small or the results are highly variable.
Researchers therefore examine both measures rather than relying on either one alone.
They may also report a confidence interval, which describes a range of values compatible with the data under the statistical method used.
Together, these measures provide a more complete picture of the estimated effect and its uncertainty.
Research discussing the distinction between statistical significance and clinical importance emphasizes that the magnitude of an effect and its practical relevance cannot be determined from statistical significance alone.
Does a Large Effect Size Mean EMDR Works for Everyone?
No. An effect size generally describes a pattern observed across participants rather than predicting an individual person's response.
For example, a study reporting a large average effect may include participants who experienced substantial improvement, others who noticed smaller changes, and some who experienced little improvement.
Effect sizes also depend on several research characteristics, including the outcomes measured, the comparison group, and the variation in participants' scores.
A large effect size compared with a waiting-list group does not necessarily mean EMDR will produce an equally large difference when compared with another established psychological therapy.
For this reason, effect sizes are most informative when interpreted within the full context of the study.
What Are the Limitations of Effect Sizes?
Effect sizes are valuable research tools, but they do not answer every question about treatment effectiveness.
Several factors influence their interpretation:
Study quality: An effect size from a study with important methodological limitations may be less reliable.
Sample size: Results from smaller studies may be less precise.
Comparison groups: Effect sizes may differ depending on whether EMDR is compared with waiting-list conditions, usual care, or another established therapy.
Outcome measures: Different assessment tools and measurement methods may produce different estimates.
Follow-up periods: An effect measured immediately after treatment may differ from one measured several months later.
Researchers also consider whether the observed difference is clinically meaningful.
Clinical significance refers to whether a measured change is important for people's symptoms, functioning, or well-being.
A statistically measurable improvement does not necessarily represent the same degree of practical benefit for every individual.
How Can Readers Interpret Effect Sizes in EMDR Studies?
When reading EMDR research, effect sizes provide useful information about the magnitude of the reported findings.
However, understanding an effect size involves looking beyond the number itself.
For example, an effect size of 0.80 may initially appear impressive under conventional guidelines. Its meaning becomes clearer when readers know which symptoms were measured, what treatment EMDR was compared against, how many people participated, and how consistently the findings were observed.
A 2025 systematic review of EMDR for adult PTSD illustrates the importance of this broader perspective. It found that EMDR performed better than waiting-list or usual-care comparisons, while differences between EMDR and trauma-focused cognitive behavioral therapy were not statistically significant in the pooled analyses.
The review also identified limitations in study quality and variation across the evidence.
Source: Clinical and cost-effectiveness of EMDR for adults with PTSD (2025).
These findings demonstrate why effect sizes are most useful when considered alongside other research evidence.
Conclusion
Effect sizes help EMDR researchers understand how large observed treatment differences or symptom changes are.
They provide important information that statistical significance alone cannot show and make it easier to compare findings across individual studies and meta-analyses.
Research on EMDR has reported meaningful effect sizes, particularly in studies examining PTSD symptoms.
However, the interpretation of those findings depends on study quality, comparison groups, and the outcomes being measured.
By understanding effect sizes, readers can evaluate EMDR research more accurately and recognize the difference between a statistically detectable result and an improvement that may be meaningful in everyday life.
References
1. Chen, Y. R., Hung, K. W., Tsai, J. C., et al. (2014). Efficacy of eye-movement desensitization and reprocessing for patients with posttraumatic-stress disorder: A meta-analysis of randomized controlled trials. PLOS ONE, 9(8), e103676. Read the study.
2. Cuijpers, P., van Veen, S. C., Sijbrandij, M., Yoder, W., & Cristea, I. A. (2020). Eye movement desensitization and reprocessing for mental health problems: A systematic review and meta-analysis. Cognitive Behaviour Therapy, 49(3), 165–180. Read the study.
3. Cochrane. Cochrane Handbook for Systematic Reviews of Interventions, Chapter 6: Choosing Effect Measures and Computing Estimates of Effect. Read the handbook.
4. Cochrane. Cochrane Handbook for Systematic Reviews of Interventions, Chapter 15: Interpreting Results and Drawing Conclusions. Read the handbook.
5. Clinical and cost-effectiveness of eye movement desensitization and reprocessing for treatment and prevention of post-traumatic stress disorder in adults: A systematic review and meta-analysis. (2025). British Journal of Psychology. Read the study.
