Sampling Method
MUS is standard for substantive overstatement testing. Classical is used when items are similar in size or when testing for understatement.
↳ ISA 530.A4 — items are selected proportional to their monetary value. Larger items have a higher probability of selection. Standard for overstatement testing.
Confidence Level
Reflects the assessed risk of material misstatement. Higher confidence = larger sample.
↳ Risk of incorrect acceptance: 10%. Confidence factor (CF): 2.31.
Population & Materiality
Enter the total monetary value of the population being sampled and the materiality thresholds from your planning.
Total monetary value of the account or class of transactions
Typically performance materiality or a fraction of it
Prior year misstatements or current estimate. Leave blank or enter 0 if none.
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ISA 530 Sample Size: How the Calculation Works
Under ISA 530, the auditor must determine a sample size sufficient to reduce sampling risk to an acceptably low level. For monetary unit sampling (MUS), this means applying a confidence factor derived from the assessed risk of material misstatement against the tolerable misstatement for the population, adjusted for any expected misstatement.
The MUS formula
The standard MUS formula is: n = (Population × Confidence Factor) / (Tolerable Misstatement − Expected Misstatement × Expansion Factor). The confidence factor reflects the acceptable risk of incorrect acceptance — at 90% confidence, the factor is 2.31. Expected misstatement increases the required sample size because the auditor must leave headroom above the expected level before reaching the tolerable threshold.
Top stratum and large items
Items above the sampling interval are automatically selected in full — they form the top stratum. This is because any single item above the interval, if misstated by 100%, would on its own produce a misstatement exceeding the interval, which when projected across the population would exceed tolerable misstatement. These items must be tested individually outside the sample.
ISA 530.5 — Audit sampling means applying audit procedures to less than 100% of items within a population so that all sampling units have a chance of selection.
ISA 530.A4 — MUS uses monetary units as the sampling unit, giving each monetary unit an equal probability of selection.
ISA 530.A11 — Sample size is affected by the tolerable misstatement, expected misstatement, and the required level of confidence.