Answer
For substantive tests, sample size increases with higher assessed risk of material misstatement, higher required assurance, and larger expected misstatement, and decreases with a larger tolerable misstatement and reliance on other procedures. For tests of controls, size increases with a lower tolerable deviation rate, higher expected deviation rate, and higher required confidence. Population size has little effect for large populations.
Substantive: higher risk / lower tolerable error -> larger sample.
Controls: lower tolerable deviation / higher confidence -> larger sample.Real-world example Because receivables are high risk with a low tolerable misstatement, the auditor selects a larger confirmation sample.