Cohen's d Calculator

Q: What is Cohen's d?

Cohen's d is a standardized measure of effect size that quantifies the difference between two group means in terms of their pooled standard deviation. It was introduced by Jacob Cohen and is widely used in psychology, education, and medical research to assess the practical significance of findings beyond statistical significance.

Q: How do you interpret Cohen's d values?

According to Cohen's conventions: d = 0.2 indicates a small effect (the groups differ by 0.2 standard deviations), d = 0.5 indicates a medium effect, and d = 0.8 or larger indicates a large effect. However, interpretation should consider the research context, as what constitutes a meaningful effect varies by field.

Q: What is the difference between Cohen's d and Hedges' g?

Hedges' g is a bias-corrected version of Cohen's d. Cohen's d tends to overestimate the population effect size in small samples. Hedges' g applies a correction factor J to adjust for this bias. For large samples (n > 20), the difference is negligible. Use Hedges' g when sample sizes are small or for meta-analysis.

Calculate Cohen's d effect size to measure the standardized difference between two group means. Features step-by-step calculations, interactive visualization, effect size interpretation, and support for both raw data and summary statistics.

Cohen's d Calculator

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Group 2 (Control/Comparison)

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About Cohen's d Calculator

Welcome to the Cohen's d Calculator, a comprehensive effect size calculator that measures the standardized difference between two group means. This tool provides Cohen's d, Hedges' g (bias-corrected), confidence intervals, and visual interpretation to help you understand the practical significance of your research findings.

What is Cohen's d?

Cohen's d is a standardized measure of effect size that expresses the difference between two means in terms of their pooled standard deviation. Introduced by statistician Jacob Cohen in 1988, it is one of the most widely used effect size measures in behavioral and social sciences.

Unlike p-values which only indicate statistical significance, Cohen's d tells you the magnitude of the difference, helping researchers and practitioners understand whether a statistically significant result is also practically meaningful.

Cohen's d Formula

Cohen's d

$$d = \frac{\bar{X}_1 - \bar{X}_2}{s_{pooled}}$$

Where:

X̄₁, X̄₂ = Means of Group 1 and Group 2
s_pooled = Pooled standard deviation

Pooled Standard Deviation Formula

Pooled Standard Deviation

$$s_{pooled} = \sqrt{\frac{(n_1-1)s_1^2 + (n_2-1)s_2^2}{n_1 + n_2 - 2}}$$

Interpreting Cohen's d

Cohen provided rough benchmarks for interpreting effect sizes, though he emphasized that context matters:

Cohen's d Value	Effect Size	Interpretation
\|d\| < 0.2	Negligible	Difference is trivial, likely not noticeable
0.2 ≤ \|d\| < 0.5	Small	Noticeable but not large in practical terms
0.5 ≤ \|d\| < 0.8	Medium	Moderate effect with practical significance
\|d\| ≥ 0.8	Large	Substantial difference between groups
\|d\| ≥ 1.2	Very Large	Dramatic difference, clearly visible

How to Use This Calculator

Choose input method: Select "Raw Data" to enter individual values, or "Summary Statistics" if you already have mean, standard deviation, and sample size.
Enter your data: For raw data, enter numbers separated by commas or spaces. For summary statistics, provide M, SD, and n for each group.
Select pooling method: Use "Pooled SD" for standard Cohen's d, or "Control Group SD" for Glass's delta.
Calculate: Click the button to see Cohen's d, Hedges' g, confidence intervals, and step-by-step calculations.
Interpret results: Review the effect size scale, distribution visualization, and interpretation guide.

Understanding Your Results

Primary Effect Size Measures

Cohen's d: The standardized mean difference using pooled SD
Hedges' g: Bias-corrected version of Cohen's d, recommended for small samples
95% CI: Confidence interval indicating the range of plausible population effect sizes

Additional Statistics

Overlap %: Approximate percentage of distributions that overlap
Cohen's U3: Percentage of Group 2 that falls below the average of Group 1

Cohen's d vs. Hedges' g vs. Glass's Delta

When to Use Each Measure

Cohen's d: Most common choice. Use when groups have similar standard deviations and sample sizes are moderate to large.
Hedges' g: Preferred for small samples (n < 20) or meta-analysis. Applies a correction factor to reduce upward bias.
Glass's delta: Use when one group is a true control or when treatment affects variability. Uses only the control group's SD.

Hedges' g Correction Formula

Hedges' g

$$g = d \times J$$

where $J = 1 - \frac{3}{4(n_1 + n_2 - 2) - 1}$

Applications of Cohen's d

Psychology and Behavioral Research

Cohen's d is the standard effect size measure for comparing experimental and control groups in psychological studies. It allows researchers to compare results across studies with different scales.

Education

Educators use effect sizes to evaluate teaching interventions. An effect size of d = 0.4 or higher is often considered educationally meaningful (Hattie's threshold).

Medicine and Clinical Trials

While other effect sizes may be preferred, Cohen's d helps communicate treatment effects in clinical research, especially for continuous outcomes.

Meta-Analysis

Effect sizes are essential for combining results across multiple studies. Hedges' g is preferred due to its bias correction.

Limitations and Considerations

Assumes normality: Cohen's d assumes approximately normal distributions in both groups
Sensitive to outliers: Extreme values affect mean and SD calculations
Equal variance assumption: Pooled SD assumes similar variance; use Glass's delta otherwise
Context matters: A "small" effect in one field may be large in another
Direction matters: Negative d means Group 2 has a higher mean than Group 1

Frequently Asked Questions

What is Cohen's d?

Cohen's d is a standardized measure of effect size that quantifies the difference between two group means in terms of their pooled standard deviation. It was introduced by Jacob Cohen and is widely used in psychology, education, and medical research to assess the practical significance of findings beyond statistical significance.

How do you interpret Cohen's d values?

According to Cohen's conventions: d = 0.2 indicates a small effect (the groups differ by 0.2 standard deviations), d = 0.5 indicates a medium effect, and d = 0.8 or larger indicates a large effect. However, interpretation should consider the research context, as what constitutes a meaningful effect varies by field.

What is the difference between Cohen's d and Hedges' g?

Hedges' g is a bias-corrected version of Cohen's d. Cohen's d tends to overestimate the population effect size in small samples. Hedges' g applies a correction factor J to adjust for this bias. For large samples (n > 20), the difference is negligible. Use Hedges' g when sample sizes are small or for meta-analysis.

When should I use Glass's delta instead of Cohen's d?

Use Glass's delta when the two groups have substantially different standard deviations, or in experimental designs where one group is a true control. Glass's delta uses only the control group's standard deviation as the denominator, which can be more appropriate when treatment effects alter variability.

What does negative Cohen's d mean?

A negative Cohen's d indicates that Group 2 has a higher mean than Group 1. The absolute value still represents the magnitude of the effect. The sign simply indicates the direction of the difference. For example, d = -0.5 means Group 2 exceeds Group 1 by half a standard deviation.

How is the pooled standard deviation calculated?

The pooled standard deviation combines the variability from both groups, weighted by their degrees of freedom. The formula is: s_pooled = sqrt(((n₁-1)s₁² + (n₂-1)s₂²) / (n₁+n₂-2)). This gives a weighted average that accounts for different sample sizes.

Additional Resources

Reference this content, page, or tool as:

"Cohen's d Calculator" at https://MiniWebtool.com/cohens-d-calculator/ from MiniWebtool, https://MiniWebtool.com/

by miniwebtool team. Updated: Jan 24, 2026

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About Cohen's d Calculator

What is Cohen's d?

Cohen's d Formula

Pooled Standard Deviation Formula

Interpreting Cohen's d

How to Use This Calculator

Understanding Your Results

Primary Effect Size Measures

Additional Statistics

Cohen's d vs. Hedges' g vs. Glass's Delta

When to Use Each Measure

Hedges' g Correction Formula

Applications of Cohen's d

Psychology and Behavioral Research

Education

Medicine and Clinical Trials

Meta-Analysis

Limitations and Considerations

Frequently Asked Questions

What is Cohen's d?

How do you interpret Cohen's d values?

What is the difference between Cohen's d and Hedges' g?

When should I use Glass's delta instead of Cohen's d?

What does negative Cohen's d mean?

How is the pooled standard deviation calculated?

Additional Resources

Related MiniWebtools:

Statistics And Data Analysis:

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