Sampling is used in the case of auditing when an auditor needs to examine a large number of transactions, documents, or account balances but cannot practically check every single item. In simple words, audit sampling allows the auditor to test a smaller, carefully selected part of a larger audit population and use the results to form a reasonable conclusion about the whole group.
In modern auditing, businesses may have thousands or even millions of invoices, receipts, payroll records, purchase orders, inventory items, or customer balances. Testing 100% of items is often too costly, too time-consuming, and sometimes unnecessary. That is why sampling in auditing is an important method for obtaining sufficient and appropriate audit evidence.
However, audit sampling is not just random guessing. It involves professional judgment, a clear sampling method, proper sample size, and careful evaluation of results. The auditor must select a representative sample, perform the right audit procedures, and consider risks such as sampling risk, non-sampling risk, tolerable misstatement, and control deviation.
This guide explains what audit sampling is, why sampling is used in auditing, where it is used, its types, examples, risks, standards, and limitations.
What Is Audit Sampling?
Audit sampling is the process of applying audit procedures to less than 100% of items within a population, so the auditor can evaluate the entire population based on the sample results. The population may be a class of transactions, an account balance, a group of invoices, payroll records, inventory items, or purchase documents.
For example, if a company has 10,000 sales invoices, the auditor may not check all 10,000 invoices. Instead, the auditor may select a sample of invoices and test whether they are recorded correctly, approved properly, and supported by valid documents.
The main elements of audit sampling include:
| Term | Simple Meaning |
| Population | The full group of items the auditor wants to test |
| Sample | The selected items tested by the auditor |
| Sampling unit | Each individual item that may be selected |
| Sample size | The number of items selected for testing |
| Representative sample | A sample that reflects the characteristics of the whole population |
A good audit sample selection helps the auditor reach reliable conclusions. A poor sample, however, can lead to erroneous conclusions, weak audit evidence, and increased audit risk.
Why Sampling Is Used in Auditing
The main reason sampling is used in auditing is that checking every transaction is usually not practical. A business may process a huge number of transactions daily. If an auditor tried to inspect every invoice, receipt, purchase order, bank transaction, payroll entry, or inventory movement, the audit would become extremely expensive and slow.
Auditing is designed to provide reasonable assurance, not absolute assurance. This means the auditor gathers enough evidence to reduce audit risk to an acceptable level, but the auditor does not guarantee that every single error or fraud will be found.
Audit sampling is useful because it helps auditors:
- Save time and audit cost
- Test large populations efficiently
- Focus on high-risk areas
- Obtain audit evidence without reviewing every item
- Support conclusions about financial statements
- Improve audit planning and audit fieldwork
For example, if the auditor believes the risk of error is higher in revenue, the auditor may use a larger sample size for sales invoices. If payroll controls appear strong, the auditor may select a smaller sample of payroll records. This is part of a risk-based auditing approach.
In short, why auditors cannot examine every single transaction comes down to practicality, cost, time, and the nature of audit assurance.
Sampling Is Used in Which Cases of Auditing?
Sampling is used in the case of auditing when the auditor needs to test a large population and draw a conclusion without checking every item. It is especially useful in tests of controls, substantive procedures, and tests of details.
| Case Where Sampling Is Used | Audit Example |
| Tests of controls | Checking whether selected purchase orders were properly approved |
| Substantive procedures | Testing selected invoice amounts for accuracy |
| Tests of details | Verifying selected account balances or transaction records |
| Accounts receivable | Sending confirmations to selected customers |
| Inventory audit | Checking selected inventory records against physical stock |
| Payroll audit | Reviewing selected employee salary records |
| Revenue audit | Testing selected sales invoices and receipts |
| Expense audit | Reviewing selected vendor invoices and payment approvals |
For instance, in a purchase audit, the auditor may select a sample of supplier invoices and check whether each invoice has a matching purchase order, goods received note, and approval. In an inventory audit, the auditor may select stock items and compare the records with the actual physical inventory.
This is why audit sampling examples with invoices, inventory, payroll records, and accounts receivable confirmations are commonly used in audit training.
Audit Sampling in Tests of Controls vs Substantive Procedures
Audit sampling can be used for different audit objectives. Two of the most important are tests of controls and substantive procedures.
| Audit Area | What the Auditor Tests | Example |
| Tests of controls | Whether internal controls are working | Was the invoice approved before payment? |
| Substantive procedures | Whether amounts are correct | Is the invoice recorded at the right amount? |
| Tests of details | Specific evidence for balances or transactions | Does the recorded sale match the customer order? |
In tests of controls, the auditor checks whether a control activity operated properly. For example, if company policy requires manager approval for all purchases above a certain amount, the auditor may select a sample of purchase orders and verify approval.
In substantive testing, the auditor checks whether financial statement numbers are accurate. For example, the auditor may test a sample of revenue transactions to confirm that sales were recorded in the correct period and at the correct amount.
This distinction matters because the auditor may consider different factors. For controls, the auditor may focus on control deviation, tolerable deviation rate, and expected population deviation rate. For substantive procedures, the auditor may focus on tolerable misstatement, expected misstatement, and material misstatement.
Key Terms in Audit Sampling
Before understanding audit sampling techniques, it is important to know the basic terms used in sampling.
Population is the complete set of items the auditor wants to test. For example, all sales invoices for the year may be the population.
Sample is the smaller group selected from the population. The auditor performs audit procedures on this sample.
Sampling unit is each individual item in the population. A sampling unit may be an invoice, payment, customer account, inventory item, or employee record.
Sample size means the number of items selected. A larger sample may be needed when the risk is higher or when the expected error rate is greater.
Margin of error refers to the possible difference between the sample result and the actual result for the full population.
Representative sample means the sample fairly reflects the population. If the sample is biased, the auditor’s conclusion may be misleading.
Tolerable misstatement is the maximum error the auditor is willing to accept in a population without changing the audit conclusion. In control testing, a similar idea is called tolerable rate of deviation.
These terms are important because audit sampling is not only about selecting items. It is about selecting the right items in the right way and interpreting the results carefully.
Types of Audit Sampling Techniques
There are several audit sampling techniques used in practice. The auditor chooses a method based on audit objective, risk level, population size, and professional judgment.
Statistical Sampling
Statistical sampling uses probability theory to select and evaluate a sample. It allows the auditor to measure sampling risk and make conclusions with more mathematical support. Random sampling and systematic sampling are often linked with statistical sampling.
This method is useful when the auditor wants a more objective basis for sample selection and evaluation.
Non-Statistical Sampling
Non-statistical sampling relies more on auditor judgment than mathematical formulas. The auditor still needs to be careful, objective, and logical, but the sample is not evaluated using formal statistical measurement.
This method is common in many audits because auditors often use experience and risk knowledge when selecting items.
Random Sampling
Random sampling means every item in the population has an equal chance of selection. For example, audit software may randomly select 100 invoices from a list of 10,000 invoices.
This supports unbiased sampling and helps reduce selection bias.
Systematic Sampling
Systematic sampling involves selecting every nth item after choosing a starting point. For example, the auditor may select every 50th invoice from a numbered list.
This method is simple and efficient, but the auditor must ensure there is no hidden pattern in the population that could distort the result.
Haphazard Sampling
Haphazard sampling is a non-statistical method where the auditor selects items without following a structured pattern. The auditor tries to avoid conscious bias, but because it depends heavily on judgment, it must be used carefully.
Judgmental Sampling
Judgmental sampling means the auditor selects items based on professional judgment. For example, the auditor may choose high-value transactions, unusual entries, or transactions from high-risk areas.
This is useful when certain items are more likely to contain errors, but it may not represent the whole population.
Monetary Unit Sampling and Attribute Sampling
Monetary unit sampling focuses on monetary amounts and is often used in substantive testing. Larger-value items have a greater chance of selection.
Attribute sampling is commonly used in tests of controls. It tests whether a specific attribute exists, such as whether an invoice was approved or whether a purchase order was matched with a receipt.
How Auditors Determine Sample Size
The auditor does not choose sample size randomly. Sample size depends on the audit objective, risk level, expected error, and the auditor’s need for evidence.
If the population has higher risk, the auditor usually selects a larger sample. If the auditor expects many errors, the sample size may also increase. If the control is very important, or the account balance is material, the auditor may need more evidence.
Key factors affecting sample size include:
| Factor | Effect on Sample Size |
| Higher audit risk | Larger sample |
| Higher expected error rate | Larger sample |
| Lower tolerable misstatement | Larger sample |
| Weak internal controls | Larger sample |
| Need for stronger evidence | Larger sample |
| Large population with similar items | May require structured sampling |
In control testing, the auditor considers tolerable deviation rate and expected population deviation rate. In substantive testing, the auditor considers tolerable misstatement, expected misstatement, and materiality.
The goal is not to test as many items as possible. The goal is to test enough relevant items to support a reliable audit conclusion.
Sampling Risk vs Non-Sampling Risk
Audit sampling always involves risk. The two major risk categories are sampling risk and non-sampling risk.
| Risk Type | Meaning | Example |
| Sampling risk | The sample result may be different from the result of testing the whole population | The sample shows few errors, but the full population has many errors |
| Non-sampling risk | The auditor makes an error unrelated to sample size | The auditor applies the wrong procedure or misreads evidence |
Sampling risk in auditing exists because the auditor is testing less than the full population. The auditor may incorrectly conclude that a population is acceptable when it is not. This is called risk of incorrect acceptance. Or the auditor may incorrectly reject a population that is actually acceptable, known as risk of incorrect rejection.
Non-sampling risk happens because of human error, poor audit planning, wrong interpretation, or weak documentation. For example, if the auditor selects a good sample but checks the wrong document, the problem is not sampling risk. It is non-sampling risk.
Both risks must be managed through good audit methodology, proper supervision, clear audit documentation, and professional skepticism.
Audit Sampling vs 100% Examination
Audit sampling is different from 100% examination. In sampling, the auditor tests less than the full population. In 100% testing, the auditor checks every item in the population.
Sampling is used when full testing is impractical. However, 100% examination may be used in certain cases, such as:
| Situation | Why 100% Testing May Be Used |
| Small population | It is easy to check every item |
| High-value transactions | Each item may be material |
| Unusual or risky items | The auditor needs more certainty |
| Automated data testing | Software can review full data sets |
| Fraud risk areas | More detailed testing may be necessary |
For example, if there are only 15 major asset purchases in a year, the auditor may test all 15 instead of selecting a sample. But if there are 50,000 sales invoices, sampling is usually more practical.
Even with modern data analytics, sampling still matters. Some audit procedures require judgment, document inspection, external confirmation, or physical verification that cannot always be fully automated.
Common Errors in Audit Sampling and What Happens If Errors Are Found
Audit sampling can be effective, but only when it is properly designed and performed. Common mistakes include choosing the wrong population, selecting a biased sample, using a sample size that is too small, or misinterpreting results.
For example, if an auditor wants to test all sales invoices for the year but only selects invoices from December, the sample may not represent the full population. That is an incorrect population definition or poor sample design.
Other common errors include weak working papers, missing documentation, ignoring exceptions, relying too much on judgment, or failing to investigate unusual items.
When errors are found in an audit sample, the auditor does not simply ignore them. The auditor may:
- Investigate the cause of the error
- Determine whether the error is isolated or systematic
- Project the error to the population
- Increase the sample size
- Perform additional audit procedures
- Reassess audit risk
- Consider the effect on the audit opinion
For substantive testing, the auditor may calculate a projected misstatement and compare it with tolerable misstatement. For control testing, the auditor may evaluate the control deviation rate and decide whether the control can still be relied upon.
Advantages and Limitations of Audit Sampling
Audit sampling offers many benefits, but it also has limitations. A balanced understanding is important for students, auditors, and business owners.
| Advantages | Limitations |
| Saves audit time | Does not provide absolute assurance |
| Reduces audit cost | Sampling risk remains |
| Helps test large populations | May miss rare errors |
| Supports risk-based auditing | Poor sample design can mislead |
| Improves audit efficiency | Judgment errors can affect results |
The biggest advantage is efficiency. Sampling helps auditors examine large volumes of data without reviewing every transaction. It also supports risk management because auditors can focus more attention on high-risk areas.
The main limitation is that sampling cannot provide complete certainty. Because the auditor tests less than 100% of items, there is always a chance that errors exist outside the sample. This is why auditors use professional judgment, proper sample design, and additional audit procedures when needed.
Can Audit Sampling Detect Fraud?
Audit sampling can help identify fraud indicators, but it does not guarantee fraud detection. Fraud may be hidden in transactions that are not selected in the sample, or it may involve management override, forged documents, or collusion.
For example, if the auditor selects a sample of vendor invoices, the sample may reveal duplicate payments, fake suppliers, or missing approvals. However, if the fraudulent transaction is outside the sample, it may not be detected through sampling alone.
That is why auditors combine sampling with other procedures, such as analytical procedures, fraud risk assessment, data analytics, interviews, and targeted testing of unusual transactions. In high-risk cases, auditors may use 100% testing or focus on specific suspicious items.
A useful quote for understanding this is: “Audit sampling supports evidence gathering, but it does not replace professional skepticism.”
Audit Sampling Standards: ISA 530, NIA 530, PCAOB AS 2315, and AICPA Guidance
Audit sampling is guided by professional auditing standards. These standards help auditors design samples, perform procedures, and evaluate results properly.
ISA 530 is the international standard that deals with audit sampling. It explains how auditors should use sampling in tests of controls and tests of details.
NIA 530 is the term commonly used in Spanish-language audit contexts for the same audit sampling standard.
PCAOB AS 2315 is important in the United States, especially for audits of public companies. It explains audit sampling, including statistical and non-statistical approaches.
AICPA audit guidance is also useful for audit sampling practices, especially in non-public company audits and professional training.
These standards emphasize that auditors must use professional judgment in planning, selecting, testing, and evaluating samples. They also highlight the importance of sample design, population characteristics, sampling risk, and clear audit documentation.
Practical Examples of Audit Sampling
Practical examples make sampling in audit easier to understand.
| Audit Area | Sampling Example |
| Sales invoices | The auditor selects invoices and checks price, quantity, date, and approval |
| Inventory records | The auditor selects stock items and compares records with physical inventory |
| Payroll records | The auditor selects employees and checks salary, tax, and attendance records |
| Accounts receivable | The auditor selects customer balances and sends confirmations |
| Purchases | The auditor selects vendor invoices and matches them with purchase orders |
| Internal controls | The auditor selects approval documents to test whether controls operated |
For example, imagine a company has 8,000 vendor invoices. The auditor selects a sample of invoices and checks whether each invoice has proper approval, correct amount, valid supplier details, and matching purchase records. If several invoices are missing approval, the auditor may investigate further and expand testing.
This type of audit sample selection helps the auditor form a conclusion without reviewing the entire population.
Short Exam-Style Answer: Sampling Is Used in the Case of Auditing
Sampling is used in the case of auditing when an auditor needs to test a large number of transactions, documents, or account balances but cannot examine every item individually. By selecting a representative sample from the full audit population, the auditor can perform audit procedures and obtain sufficient and appropriate audit evidence.
Sampling is commonly used in tests of controls, substantive procedures, and tests of details. It helps auditors save time, reduce audit cost, and focus on important or high-risk areas. However, audit sampling involves sampling risk, because the sample may not perfectly represent the whole population.
Therefore, auditors must choose the right sampling method, determine a suitable sample size, and evaluate errors carefully before forming an audit conclusion.
FAQs About Sampling in Auditing
Why do auditors use sampling instead of checking every transaction?
Auditors use sampling because checking every transaction is often impractical, costly, and time-consuming. Sampling allows auditors to obtain reasonable assurance by testing selected items from a larger population.
What are the main types of audit sampling?
The main types include statistical sampling, non-statistical sampling, random sampling, systematic sampling, haphazard sampling, judgmental sampling, monetary unit sampling, and attribute sampling.
Is audit sampling always allowed?
No. Sampling may not be suitable when the population is small, when every item is high value, or when the auditor needs to test all items because of high risk. In such cases, 100% testing may be more appropriate.
What is sampling risk in auditing?
Sampling risk is the risk that the auditor’s conclusion based on a sample is different from the conclusion that would be reached by testing the entire population.
What is non-sampling risk?
Non-sampling risk is the risk of reaching a wrong conclusion because of human error, poor audit procedures, misinterpretation, or weak documentation.
Can audit sampling find fraud?
Audit sampling may detect fraud indicators, but it cannot guarantee fraud detection. Auditors often use sampling along with data analytics, targeted testing, and professional skepticism.
Conclusion
Sampling is used in the case of auditing because it helps auditors test large populations efficiently while still obtaining useful audit evidence. Instead of checking every transaction, auditors select a representative sample, perform audit procedures, evaluate results, and form a reasonable conclusion.
The key to effective audit sampling is proper planning. Auditors must understand the population, choose the right sampling technique, determine an appropriate sample size, and evaluate errors carefully. They must also consider sampling risk, non-sampling risk, tolerable misstatement, control deviation, and relevant audit standards such as ISA 530 and PCAOB AS 2315.
In short, audit sampling is not a shortcut. It is a structured, professional method that supports reliable auditing when testing every item is not practical.
Disclaimer: This content is for general educational purposes only and does not constitute professional auditing or financial advice. Procedures, standards, and interpretations may vary by jurisdiction. Always consult a qualified auditor or refer to official auditing standards before making decisions.