# Sample size and statistical power: a defensible justification

Decision workbook and teaching example · 2026-10-06

The completed case is fictional. Its values and decisions are not actual findings; complete your project using relevant permissions, methods and requirements.

- Project: ____________________
- Researcher and role: ____________________
- Date and version: ____________________
- Data seen before the decision: ____________________
- Aim and scope: ____________________

## Completed example

| Stage/decision | Example | Check or action |
| --- | --- | --- |
| Goal | Power or precision for the main outcome | Does it match the design? |
| Analyzable need | 100 is an exercise assumption | Was the actual need properly calculated? |
| 15% attrition | Approximate recruitment 118 | Is the rate justified? |
| 25% attrition | Approximate recruitment 134 | Is the harder scenario covered? |
| Structure | Possible clusters or repetition | Are independence assumptions defensible? |

## Complete your own record

| Topic | Actual decision and reason | Evidence link and status |
| --- | --- | --- |
| Goal | __________ | __________ |
| Analyzable need | __________ | __________ |
| 15% attrition | __________ | __________ |
| 25% attrition | __________ | __________ |
| Structure | __________ | __________ |

## Completion checks

- [ ] Save calculation inputs, scenarios and the design rationale.

## Changes and limitations

| Date | Change | Data known at decision time | Reason | Consequence for claim | Reviewer |
| --- | --- | --- | --- | --- | --- |
| __________ | __________ | __________ | __________ | __________ | __________ |

## Sources and further reading

- [UCLA Statistical Methods and Data Analytics — G*Power](https://stats.oarc.ucla.edu/other/gpower/)
- [American Statistical Association — Statement on p-values](https://www.amstat.org/asa/files/pdfs/p-valuestatement.pdf)

Related guide: https://didgar-research-site1.vercel.app/en/guides/sample-size-power/