The best situation that justifies the use of a smaller sample size when testing the accuracy of transaction data is B. It is expected that the population is error-free. The sample size is the number of items selected from the population for testing. The sample size depends on various factors, such as the level of confidence, the tolerable error rate, the expected error rate, and the variability of the population. A smaller sample size means that fewer items are tested, which reduces the cost and time of testing, but also increases the sampling risk (the risk that the sample is not representative of the population).

One of the factors that affects the sample size is the expected error rate, which is the auditor’s best estimate of the proportion of errors in the population before testing. A higher expected error rate means that more errors are likely to be found in the population, which requires a larger sample size to provide sufficient evidence for the auditor’s conclusion. A lower expected error rate means that fewer errors are likely to be found in the population, which allows a smaller sample size to provide sufficient evidence for the auditor’s conclusion. Therefore, if it is expected that the population is error-free (i.e., the expected error rate is zero or very low), a smaller sample size can be justified.

The other situations do not justify the use of a smaller sample size when testing the accuracy of transaction data. A. The IS audit staff has a high level of experience. The IS audit staff’s level of experience does not affect the sample size, but rather their ability to design and execute the sampling procedures and evaluate the results. The IS audit staff’s level of experience may affect their judgment in selecting and applying sampling methods, but it does not change the statistical or mathematical principles that determine the sample size. B. Proper segregation of duties is in place. Proper segregation of duties is an internal control that helps prevent or detect errors or fraud in transaction processing, but it does not affect the sample size. The sample size is based on the characteristics of the population and the objectives of testing, not on the controls in place. Proper segregation of duties may reduce the likelihood or impact of errors or fraud in transaction processing, but it does not eliminate them completely. Therefore, proper segregation of duties does not justify a smaller sample size when testing the accuracy of transaction data. C. The data can be directly changed by users. The data’s ability to be directly changed by users does not justify a smaller sample size, but rather a larger one. The data’s ability to be directly changed by users increases the risk of errors or fraud in transaction processing, which requires a larger sample size to provide sufficient evidence for the auditor’s conclusion. The data’s ability to be directly changed by users also increases the variability of the population, which affects the sample size.