Audit Sampling in IS Audit
SA 530 still applies. In an IS audit, the first question is often whether to sample at all.
Sampling in an IS audit follows the same standard as in a financial audit, SA 530, but the populations look different: user access requests, program changes, firewall rule changes, privileged log-ins, password resets. Most IS control tests are attribute tests (did the control operate or not?), so the measure is a rate of deviation rather than a rupee misstatement.
IS audit also gives you an option a manual audit rarely has. When the population is fully electronic, a CAAT can test every item. SA 500 recognises selecting all items as a valid alternative to sampling, so the first question is often whether to sample at all.
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The SA 530 Terms You Need
- Audit sampling
- Applying procedures to less than 100% of a population so that every sampling unit has a chance of selection, to conclude on the whole population.
- Statistical sampling
- Random selection plus probability theory to evaluate results and measure sampling risk. Missing either feature makes it non-statistical. Sample size is not what distinguishes the two.
- Sampling risk
- The risk that a conclusion from the sample differs from what testing the whole population would show. Concluding controls are more effective than they are hurts audit effectiveness; concluding they are less effective hurts efficiency.
- Non-sampling risk
- Wrong conclusions for reasons unrelated to sampling, such as an unsuitable procedure or misreading the evidence. Judgmental selection of specific items carries this risk.
- Tolerable rate of deviation
- The deviation rate the auditor sets for a control test, above which the control cannot be relied on.
- Anomaly
- A deviation shown to be not representative of the population. SA 530 treats this as extremely rare and requires additional procedures for a high degree of certainty.
What Moves Sample Size for a Test of Controls
From SA 530's Appendix 2. The direction matters more than any number.
Reliance placed on the control
Sample size
Increases
Why
More assurance needed from the control's operation
Tolerable rate of deviation
Sample size
Decreases
Why
A looser threshold needs less evidence
Expected rate of deviation
Sample size
Increases
Why
More items needed to estimate the actual rate
Desired level of assurance
Sample size
Increases
Why
Higher confidence demands more evidence
Number of items in a large population
Sample size
Negligible effect
Why
Population size barely matters once it is large
| If this increases | Sample size | Why |
|---|---|---|
| Reliance placed on the control | Increases | More assurance needed from the control's operation |
| Tolerable rate of deviation | Decreases | A looser threshold needs less evidence |
| Expected rate of deviation | Increases | More items needed to estimate the actual rate |
| Desired level of assurance | Increases | Higher confidence demands more evidence |
| Number of items in a large population | Negligible effect | Population size barely matters once it is large |
Selection Methods and Where They Fit
SA 530 names these as the principal methods.
- check_circleRandom selection: each item has a known chance; suits statistical sampling of, say, user-creation tickets from the IT service desk.
- check_circleSystematic selection: a fixed interval from a random start. Check that the population has no pattern that lines up with the interval (for example, weekly batch jobs and an interval of seven).
- check_circleMonetary unit sampling: value-weighted selection for tests of details, giving conclusions in monetary terms.
- check_circleHaphazard selection: no structured technique, but no conscious bias. Not appropriate for statistical sampling.
- check_circleBlock selection: contiguous items, such as all changes in March. SA 530 says this is rarely appropriate when you mean to draw conclusions about the whole population.
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Picking the Risky Items Is Not Sampling
Selecting all privileged-user changes or every journal over ₹1 crore is a sound audit procedure, but under SA 500 it is selecting specific items, not audit sampling. You cannot project the result to the rest of the population. ICAI's Module 1 material loosely lists judgmental sampling as a non-statistical method; for the standard's own position, rely on SA 500 and SA 530.
Handling Exceptions
SA 530 requires the auditor to investigate the nature and cause of every deviation. If a selected item cannot be tested (the change ticket is missing), it counts as a deviation; you do not quietly swap it for another. A replacement is allowed only when the procedure is genuinely not applicable to that item, such as a cancelled request that was never processed.
In an access review at a mid-size company on SAP, one terminated employee still holding an active ID may look small. But if the cause is that HR exits never reach IT, the deviation is systemic, and the tolerable rate is probably exceeded.
How the DISA Assessment Test Tests This
- check_circleDirection questions: "if the tolerable deviation rate is lowered, sample size..." (increases). Candidates mix up tolerable and expected rates.
- check_circle"What distinguishes statistical from non-statistical sampling?": random selection and probability-based evaluation, not sample size.
- check_circle"Which risk leads to over-reliance on controls?": sampling risk of the first type, the one that affects audit effectiveness.
- check_circleScenarios where a CAAT over the full population beats any sample, especially duplicate or sequence tests on ERP data.
FAQs
What is the difference between statistical and non-statistical sampling?expand_more
Statistical sampling uses random selection and probability theory to evaluate results and measure sampling risk. Without both, it is non-statistical, regardless of sample size.
Is 100% testing allowed instead of sampling?expand_more
Yes. SA 500 lists selecting all items as one means of selecting items for testing. It is common in IS audit where data is electronic and a CAAT makes full-population testing cheap.
What is tolerable rate of deviation?expand_more
The rate of control deviation the auditor sets as acceptable. If the actual rate in the population may exceed it, the control cannot be relied on as planned.
Can I replace a sample item that has no documentation?expand_more
No. Under SA 530, if the designed or an alternative procedure cannot be applied, the item is treated as a deviation or misstatement.
Next steps
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