Another sensitivity question is whether failure to adhere to the optimal audit interval increases costs significantly. This is detennined by the impact on total discounted costs of deviations of the audit interval from the optimal interval. It can be shown by differentiation of the total discounted cost function with respect to the audit interval, /„, that the impact of ?„ on total costs depends on complicated interactions between each of the variables, C and M, and the parameters, r and p, in the analysis (see Boritz and Broca [1986, p. 11]). A simulation approach is therefore useful. We compute the average monthly discounted costs over audit intervals from 1 month to 24 months, using a long finite planning horizon (120 months). The results are shown in Figure 6. The first point to observe is that the TDC follow a skewed (to the right) U shape. The skewing to the right is the result of discounting and therefore applies to all firms in the sample. This means that for our data and parameter values, the net gain from increasing the audit interval is large for very small intervals until the minimum is reached. Beyond the minimum point, the net loss from increasing the audit interval becomes increasingly small. The different rates of net gain and net lo.ss as the audit interval is increased is a result of both the data—audit fees and asset size—and the values chosen for the parameters—the interest rate and the beta scaling factor. (Sensitivity of the results to these factors has been discussed above.) The skewed shape
means that the cost of having an audit a given number of months too often is higher than the cost of auditing the same number of months too infrequently. The costs of having an audit 3 months too soon and 3 months too late are given in Table 2 for a sample of four tirms. The sample was chosen to consist of firms which had a range of optimal audit intervals, being 3, 8, 12, and 21 months, respectively. The first point is that the costs of erring differ greatly between firms. Second, the table illustrates the point that it is less costly to audit too infrequently than it is to audit too frequently. Third, the lower the optimal audit interval the greater the costs of auditing too soon or too late by a given time interval (3 months in this example).
Another sensitivity question is whether failure to adhere to the optimal audit interval increases costs significantly. This is detennined by the impact on total discounted costs of deviations of the audit interval from the optimal interval. It can be shown by differentiation of the total discounted cost function with respect to the audit interval, /„, that the impact of ?„ on total costs depends on complicated interactions between each of the variables, C and M, and the parameters, r and p, in the analysis (see Boritz and Broca [1986, p. 11]). A simulation approach is therefore useful. We compute the average monthly discounted costs over audit intervals from 1 month to 24 months, using a long finite planning horizon (120 months). The results are shown in Figure 6. The first point to observe is that the TDC follow a skewed (to the right) U shape. The skewing to the right is the result of discounting and therefore applies to all firms in the sample. This means that for our data and parameter values, the net gain from increasing the audit interval is large for very small intervals until the minimum is reached. Beyond the minimum point, the net loss from increasing the audit interval becomes increasingly small. The different rates of net gain and net lo.ss as the audit interval is increased is a result of both the data—audit fees and asset size—and the values chosen for the parameters—the interest rate and the beta scaling factor. (Sensitivity of the results to these factors has been discussed above.) The skewed shape
means that the cost of having an audit a given number of months too often is higher than the cost of auditing the same number of months too infrequently. The costs of having an audit 3 months too soon and 3 months too late are given in Table 2 for a sample of four tirms. The sample was chosen to consist of firms which had a range of optimal audit intervals, being 3, 8, 12, and 21 months, respectively. The first point is that the costs of erring differ greatly between firms. Second, the table illustrates the point that it is less costly to audit too infrequently than it is to audit too frequently. Third, the lower the optimal audit interval the greater the costs of auditing too soon or too late by a given time interval (3 months in this example).
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