ADHD glossary · Family perspectives
Effect size: how much, measured in what?
The Cochrane Handbook notes that “effect size” is sometimes used specifically for a standardized mean difference and sometimes more generally for an effect estimate, so its meaning depends on the measure being reported. [1]
General education and practical perspectives. Editorial standards.
Our first question is not whether the number looks impressive. It is what the number measures. A difference in minutes, a ratio and a standardized score do not share a single scale.
An invented two-minute difference
Suppose a fictional description gives the average time for two groups to finish a paper-sorting activity: 14 minutes for group A and 12 minutes for group B. Subtracting those displayed averages gives A minus B equal to two minutes.
That arithmetic describes the two reported averages. It does not tell us why they differ, whether people were assigned to groups, how variable their times were or whether the same contrast would appear elsewhere. We have not supplied those details.
If the writer reverses the order to B minus A, the difference becomes minus two minutes. The sign changed because the subtraction order changed. It is not a separate discovery. Keep the group labels next to the number when passing the example along.
Units before adjectives
Try making a short result card:
- What was measured?
- Which groups or conditions are compared, and in what order?
- Which measure is reported?
- What are its units, if any?
- What uncertainty is reported?
- What would be needed to judge its relevance to this particular task?
For the invented sorting example, “two minutes” is more informative than “a positive effect.” It still does not justify calling the difference valuable to every participant. We do not know what they wanted from the activity or what else occurred.
The question can change by perspective
A teenager might care about having time to finish a project they chose. A parent might focus on how directions were delivered. A grandparent might ask whether the result came from people in a comparable setting. An educator may need to know whether the outcome was completion time or accuracy.
Those are not competing interpretations of one bare number. They identify context that the number alone does not contain. Nobody's personal priority has to become the universal definition of a meaningful result.
Keep magnitude separate from certainty
A report may include an effect estimate, an uncertainty interval and a statistical test. Read their labels separately rather than replacing the whole result with “it worked.” If the summary omits the measure, leave the interpretation open until the original report supplies it.
We do not use this page to recommend an intervention, attach a clinical meaning to a score or supply universal thresholds for small and large effects. The practical task is to preserve the measure, units, comparison and uncertainty whenever we repeat a numerical claim.
Source
[1] Julian P. T. Higgins; Tianjing Li; Jonathan J. Deeks. Chapter 6: Choosing effect measures and computing estimates of effect. Cochrane Handbook for Systematic Reviews of Interventions. Chapter citation says last updated August 2023, Handbook version 6.5 (2024); exact publication day not stated. Accessed September 6, 2026. Sections 6.1.2 and 6.5.1.2, including the multiple uses of effect size. Supports the adjacent attributed claim. All illustrative scenes, numbers and optional note formats are original inventions, not source findings.
Related guides
Sources and what they support
These sources support the specific statements identified below. Our practical exercises are suggestions to consider, not evidence of a treatment effect.
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Chapter 6: Choosing effect measures and computing estimates of effect — Cochrane Handbook for Systematic Reviews of Interventions
Source type: institutional.
Accessed .
Source check, September 6, 2026: Scoped source check. Passage: Sections 6.1.2 and 6.5.1.2, including the multiple uses of effect size. Limits: Measurement terminology only. No small/large benchmarks, intervention findings or treatment interpretation adopted.
Supports: The Cochrane Handbook notes that “effect size” is sometimes used specifically for a standardized mean difference and sometimes more generally for an effect estimate, so its meaning depends on the measure being reported.
Correction check: The identified original passage, surrounding context and available bibliographic material were inspected on September 6, 2026. No notice changing the adopted definition was found in that material. This is not a complete publisher-history, Crossmark, correction-database or retraction clearance.
Updates and corrections
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