The Power of Random Sampling in Audits: Methodology, Formula & Compliance Value

1. Introduction

Audits are a cornerstone of corporate governance and compliance. But when faced with large volumes of data or transactions, auditing every item is rarely practical. This is where random sampling becomes a powerful tool — enabling auditors to assess compliance, detect irregularities, and draw reliable conclusions efficiently.

At Crest Advisory Africa, we train and support internal audit functions across sectors on the strategic use of random selection in audit planning and execution. In this article, we explain how to calculate and apply random selection formulas in line with ISO 19011 and ISO 37301 audit frameworks.

2. Why Use Random Sampling?

Random sampling ensures that the items selected for an audit:

  • Are free from auditor bias
  • Provide a representative view of the population
  • Support objective evidence gathering
  • Save time and cost compared to full-population audits

Standards such as ISO 37301 (Compliance Management Systems) and ISO 19011 (Audit Guidelines) recommend using statistically sound sampling methods when 100% review is not feasible.

3. The Sampling Formula

To calculate the sample size based on a percentage of a dataset, use:

✅ Example:

  • If you have a dataset of 300 records and wish to audit 30% of them:

  • You need to randomly select 90 records for your audit.

4. How to Perform Random Selection in Excel

You can easily perform random sampling using Microsoft Excel:

  • Enter your data into Column A (e.g., A1:A300).
  • In Column B, next to each entry, enter:

  • This will generate a random number between 0 and 1 for each row.
  • Sort the data based on Column B (ascending or descending).
  • Select the top 90 rows (30%) as your audit sample.
  • Optional: Flag the Selected Items
  • Use this formula in Column C to label the selected records:

  • This formula ranks each record and flags the top 90 as "SELECTED".

5. When to Use Random Sampling

  • Internal audits of expense claims, procurement records, or contract reviews
  • ISO 37001 anti-bribery audits to review a subset of high-risk transactions
  • Compliance audits where population size exceeds practical review limits
  • Due diligence and investigations requiring impartial review samples

6. Crest Advisory Africa's Support Tools

Crest Advisory Africa provides:

✅ ISO 37301 and ISO 19011-compliant Audit Training

✅ Custom-built audit programs with integrated sampling methods

✅ Excel templates and ISOLTX GRC system modules to manage random sampling, evidence collection, and audit reporting

✅ Audit planning advisory to determine sample size, risk stratification, and audit frequencies

7. Downloadable Audit Sampling Template

We've created an Excel-based Random Audit Selection Tool that automates:

  • Sample size calculation
  • Random number generation
  • Selection flagging

Click here to download the template

8. Conclusion

Random selection isn't just a technical tool — it's a safeguard of audit objectivity and a promoter of integrity. It supports fair, risk-aligned, and ISO-compliant assurance activities.

At Crest Advisory Africa, we help organisations implement smart, evidence-based auditing practices that strengthen governance and trust.

www.crestadvisoryafrica.com

info@crestadvisoryafrica.com