CISA Information System Auditing Process Practice Question
An auditor is selecting a sample of purchase orders for testing. The auditor decides to select every 50th purchase order from a list. This is an example of:
⚠ Common exam trap
CISA often tests the distinction between systematic and random sampling — candidates see 'every 50th' and think it's random, but the fixed interval is the defining characteristic of systematic sampling.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
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Systematic sampling
Systematic sampling involves selecting every nth item from a sequentially ordered population, such as every 50th purchase order. This method is a statistical sampling technique that provides a random-like selection if the starting point is random and the population is not patterned. The scenario explicitly describes selecting every 50th item, which is the definition of systematic sampling.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Judgmental sampling
Why it's wrong here
Judgmental sampling relies on the auditor's professional discretion to pick items based on risk or materiality, so it cannot explain a fixed interval selection. It is tempting because it suits targeted testing of high-value or suspicious transactions, but here the mechanical every-50th rule defines systematic sampling instead.
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Random sampling
Why it's wrong here
Selecting every 50th purchase order is systematic selection, which uses a fixed interval through the population; random sampling gives every item an equal chance via random number generation. Systematic sampling is tempting because the interval feels arbitrary, but the deterministic pattern is the defining difference.
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Systematic sampling
Why this is correct
Systematic sampling selects items at fixed intervals from a population list, here every 50th purchase order. This satisfies the auditor's need for an efficient, unbiased selection method that spreads the sample evenly across the entire sequence, provided the list has no cyclical pattern aligning with the interval.
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Stratified sampling
Why it's wrong here
Stratified sampling divides the population into distinct subgroups and samples from each; selecting every 50th item applies one fixed interval across the whole list without stratification. It is tempting because both are probability methods, but no subpopulation grouping occurs here, so the technique is systematic selection.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official ISACA exam blueprint
This CISA practice question is part of Courseiva's free ISACA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CISA exam.