If your business is preparing for certifications like ISO 9001 or ISO 27001, conducting an internal audit by using the right audit sampling method will help the organization assess its quality management systems for compliance.
Audit sampling typically involves reviewing about 10 to 20% of total transactions or records to gather sufficient evidence for ISO 9001 certification audits. Let's look at the audit sampling methods that help your organization confidently pass the next certification audit.
What is an Internal Audit?
Internal audits are systematic reviews conducted within a company to check for compliance with the operations and processes involved. These audits also help the company to identify risks and suggest improvements in the operations to work at their best capacity.
For example, an internal audit may review how the company handles its customer complaints and accordingly suggest improvements.
Some of the certifications that require internal audits are:
- ISO 9001 (Quality Management Systems)
- ISO 27001 (Information Security Management)
- ISO 14001 (Environmental Management Systems)
What is Audit Sampling?
Audit sampling is a process implemented by auditors to select and examine a subset that represents a larger set of records or transactions. Instead of the tedious task of reviewing every item, auditors sample data to gather reliable audit evidence efficiently and provide conclusions for the entire population. In audit sampling, a population means the entire group of records or transactions from which an auditor selects a sample that represents the entire set.
For example, during an ISO 9001 audit, it's unrealistic to check every invoice. Hence, a set of chosen samples is tested to infer compliance.
Internal audit sampling ensures:
- Efficient use of audit time and resources
- Reliable gathering of audit evidence
- Objective evaluation of compliance and controls
What are Audit Sampling Methods?
How to choose the samples? What factors should be taken into consideration when sampling for compliance? Sampling methods answer these questions and determine how samples are selected.
Internal auditors use either probability or non-probability sampling methods to select items. The choice depends on the audit objective, population size, and risk considerations. Let's break down both main categories.
Probability Audit Sampling
In probability audit sampling, every item in the population gets a defined chance of being selected. This method is often preferred for ISO 9001 audits.
1. Cluster Sampling
Cluster sampling sorts out the population into groups, also known as clusters, and entire clusters are randomly selected for audit. It's suitable when transactions are naturally grouped, such as by location or region.
For example, auditing three randomly selected warehouses out of ten.
2. Systematic Sampling
In systematic sampling, internal auditors select every nth item from an ordered list, for example, every 5th invoice of a batch.
Systematic sampling is straightforward, unbiased, and ideal for large, well-ordered datasets.
3. Stratified Random Sampling
In this type of sampling, the population gets split into groups, also known as strata. On the basis of risk or importance, random samples are drawn within each group. An intensive risk-based audit approach is implemented to sample the higher-risk strata.
More samples are drawn from high-value transactions than from low-value ones.
Non-probability Audit Sampling
Non-probability sampling is based on the auditor's judgement. Not every item has a defined chance of selection; hence, even if this is a quicker and easier method, it can still introduce bias and is generally less robust for certification purposes.
1. Convenience Sampling
The internal auditor selects items that are easiest to access or review. This method saves time but is less likely to uncover hidden issues.
For example, examining records for the most recent month.
2. Judgment Sampling
Here, the auditor uses experience and knowledge to target items most likely to reveal meaningful audit evidence. This method is commonly used in risk-based audit work for areas of known risk or those with a history of previous issues.
For example, selecting transactions near quarter-end, according to historical data, is where errors occur the most.
3. Quota Sampling
The auditor specifies specific characteristics (such as department or region) and ensures that each is represented proportionally in the sample.
Here, it is ensured that every business unit in an organization is sampled.
4. Snowball Sampling
This type of sampling is used for a population that is tricky to comprehend. Snowball sampling involves auditing certain items, and then the auditor lets those choices lead to the next step (such as referrals). It's rare in ISO 9001 audits, but it might be used for compliance investigations.
Understanding Sampling Risks and Standards
Audit sampling is not without risk. The two main risks are:
Sampling risk: This is when the sample does not represent the population, leading to incorrect conclusions.
Non-sampling risk: These errors are not related to sampling, such as misinterpreting evidence or using an inappropriate audit procedure.
To minimize sampling risk, the audit must utilize sufficiently large and representative samples and follow a well-defined method, particularly for ISO 9001 audit compliance.
Standards Governing Sampling in Auditing
Major standards for sampling that are included in auditing are as follows.
- Standard on Internal Audit (SIA) 5 emphasizes designing samples that meet audit objectives, selecting methods aligned with the population, and evaluating results by projecting errors found in the sample to the population.
- SA 530: Defines audit sampling, sample size determination, and risk management. Internal auditors must design sampling rules so that every item in the population has a reasonable chance of getting selected for the sample.
Summing Up
Picking the ideal sampling method is an essential decision for passing ISO 9001 and other quality certifications. In this case style, we saw that probability sampling offers statistical rigor and is generally favored for objective audit evidence. On the other hand, non-probability sampling can be faster but should be used with caution, especially for critical or high-risk audits.
Businesses must always align their audit approach with relevant standards and clearly document the rationale behind their sampling.
Effivity empowers organizations to simplify, automate, and optimize every aspect of internal audit sampling for ISO compliance. It makes every step – from planning to implementing solutions – audit-ready with the help of an organized workflow.
With Effivity, it is easy to:
- Automate audit scheduling and notification for multiple sites or teams.
- Tailor custom checklists and sampling methods aligned to ISO standards.
- Track audit findings, initiate corrective actions, and monitor their effectiveness in real time.
- Allowing all stakeholders to securely review, approve, and close audits from a unified platform
Effivity's flexible audit solutions enhance your sampling strategy, documentation, and corrective actions, making your business pass the toughest certification audits with ease.
Start your journey with Effivity today and make every audit easier, faster, and more reliable.
