A spreadsheet is not low impact just because it is familiar
The important questions are what it calculates, how the result is used, who can alter it, and how errors are detected. A workbook without macros can still contain an incorrect formula, hidden row, broken reference, or misleading rounded result.
Four different uses
| Use | Example concern | Proportionate focus |
|---|---|---|
| Personal task list | Missed reminder | Business-use assessment and appropriate information protection |
| Quality metrics | Wrong denominator changes a reported rate | Data definitions, source completeness, formula, review, version |
| Acceptance calculation | Incorrect result supports product disposition | Independent calculation, specification rules, input controls, protection, review |
| Reliability analysis | Statistical assumptions do not fit the data | Approved method, assumptions, independent reference, units, interpretation |
Worked calculation: dilution-adjusted result
Fictional teaching example. A workbook calculates concentration as instrument reading multiplied by dilution factor. A reading of 2.5 mg/L and factor 4 gives 10.0 mg/L.
Check more than that ordinary case. What happens if the dilution factor is blank, zero, negative, entered as text, or copied from the wrong sample? What if a user pastes over the formula? What if the specification is stored in different units? What happens when a row is inserted?
Use an independently checked expected result. Verify both the calculation and the association of the result with the correct sample. A correct number against the wrong sample can still cause the wrong decision.
Completed assurance record extract
Workbook QC-CALC-02, revision 3, calculates concentration for the specified analytical method. Formula cells and controlled reference data are protected. Approved inputs are reading, dilution factor, and sample identifier. Verification covers ordinary values, permitted limits, invalid and missing inputs, formula integrity, row-copy behavior, units, and specification comparison. Use is limited to the defined method. A changed formula, method, specification rule, or relevant software behavior triggers assessment.
Protection reduces accidental modification; it is not automatically a complete access-control or tamper-prevention strategy. Assess file permissions, distribution, approved versions, and record retention as well.
Does 100% checking remove the need for assurance?
A complete independent check can be an important control, but its effectiveness must be demonstrated. Define what is checked: inputs, formulas or independent method, results, units, decisions, and record identity. Establish who performs the check and when.
For one isolated calculation, an approved independent calculation and review may support the use. For a recurring workbook used hundreds of times, reliance on manual recalculation may be error-prone and expensive. Evaluate the recurring process and maintain the controlled template. “Every number is checked” is not an automatic exemption.
Reliability example: MTTF confidence bound
A workbook may report a lower confidence bound on mean time to failure. Before checking its formula, define the statistical model, failure-time assumptions, censoring, exposure units, confidence level, and treatment of zero failures. A formula correct under one lifetime model may be unsuitable for another dataset.
A useful test compares the workbook with an independently established reference for the approved method, including a zero-failure case if the method supports it, a case with observed failures, and invalid input. Do not infer statistical suitability from matching a single calculator output.
Exercise: An analyst duplicates a validated workbook and changes an acceptance limit for another product. Can the original conclusion simply be reused?
Answer: Assess the new intended use, limit source, units, comparison rule, and changed cells. Existing evidence may still support unchanged logic, but the new configuration and product application need a justified review.