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Effect Size (Difference) Calculator

Simple magnitude of difference between two means.

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Difference

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Overview

The raw effect size represents the absolute difference between the means of two distinct groups, such as an experimental and a control group. Unlike standardized metrics, this calculation preserves the original units of measurement, allowing for direct interpretation of the magnitude of change.

Symbols

Variables

D = Difference, M_E = Exp. Mean, M_C = Ctrl. Mean

Difference
Exp. Mean
Ctrl. Mean

Apply it well

When To Use

When to use: Apply this formula when comparing two groups on a continuous dependent variable where the measurement scale has intrinsic, well-understood meaning. It is ideal for metrics like reaction time in milliseconds, blood pressure in mmHg, or scores on a validated psychometric scale.

Why it matters: This calculation shifts the focus from simple statistical significance (p-values) to practical significance. It helps researchers and practitioners determine if the size of an intervention's impact is large enough to justify its implementation in real-world settings.

Avoid these traps

Common Mistakes

  • Assuming a large difference is always statistically significant.

One free problem

Practice Problem

A clinical psychologist evaluates a new anxiety treatment. The experimental group (M1) has a mean post-treatment score of 22, while the control group (M2) has a mean score of 35. Calculate the raw effect size (D).

Exp. Mean22
Ctrl. Mean35

Solve for:

Hint: Subtract the control group mean from the experimental group mean.

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Wikipedia: Effect size
  2. Gravetter, F. J., & Wallnau, L. B. (2017). Statistics for the Behavioral Sciences (10th ed.). Cengage Learning.
  3. American Psychological Association (APA) Publication Manual, 7th Edition
  4. Field, A. (2018). Discovering Statistics Using IBM SPSS Statistics (5th ed.). SAGE Publications.
  5. Wikipedia: Effect size (accessed 2023-10-27)
  6. Wikipedia article 'Effect size'
  7. Wikipedia article 'Levels of measurement'
  8. Wikipedia article 'Independence (probability theory)'