Table Four shows more complete data about Michigan industries. Table Four also shows the SBT burden as a fraction of Business Income (profit), in order to establish some comparison between the burden placed by the SBT and a (hypothetical) equivalent profits tax.

Of course, part of the rationale of the SBT is that value-added is a better tax base than profit. Because profit serves as the tax base for most business tax systems, (including the federal corporate income tax, which provides the starting point for calculating the SBT), businesses usually have an incentive to reduce declared profits in order to reduce tax liability. Similarly, Michigan businesses have an incentive to reduce portions of their operations (such as payroll) that contribute to the SBT base. Thus, direct comparisons should be viewed with caution, since businesses will change their operations to lower their tax liabilities.

The 1981 DMB study compared the SBT with a hypothetical state corporate income tar. The study found that an equivalent revenue would have been raised from a 9.7% corporate income tax (with no exemptions) as from the SBT in 1977-78. [39] As noted above, this static analysis ignores the behavioral changes firms would make if actually subjected to a corporate income tax. Furthermore, it ignores non-corporations.

Besides the conceptual problems in static comparisons of tax systems, there are two difficulties with the calculation in Table Four. First, for many industries Business Income was negative during the period. Thus, a simple ratio would have a negative denominator. As a correction, the table constrains the ratio to be nonnegative and less than or equal to one. For industries that lost money but paid SBT, the ratio reads 1.000, meaning that the industry paid 100% of its (nonexistant) profit in SBT.

This method has the advantage of showing a high tax burden ratio for industries that lost money but paid taxes. In addition, the carryforward provisions of most income tax systems (and the SBT) allow a firm with a negative tax base in one year to carryforward the excess to the next year, thus offsetting some positive tax base in the future. The use of "tax loss carry forwards" thus creates the opportunity for eventual 100% offset of loss.

The second difficulty with the comparison of Business Income with SBT Liability is the overestimate of Business Income contained in the data. While the other data in this study refer to firms in the SBT sample, which contains 95% of all firms, the Business Income data refers to all SBT filers. [40] The Treasury states that the Business Income data are unavailable for the sample alone.

Therefore, comparing Business Income of all firms with the SBT Liability of only the firms in the sample yields an underestimate of the fraction of profits paid in SBT. Given these two difficulties, the SBT/Business Income ratio should be used with caution. That notwithstanding, the data yields some startling results.

The first result is the extreme variation in the fraction of profits paid in SBT, ranging from about 7% for Non-Durable Manufacturers to over 100% for Agriculture, Transportation and Transportation Equipment, Communication & Utilities, and Finance, Real Estate & Insurance.

The second result is the startlingly high percentage of the total Business Income of Michigan businesses paid in SBT. Even with the overestimate of Business Income contained in the data, the ratio of SBT Liability to Business Income exceeds 20%!

These results illustrate the points made earlier about the cyclical intensity of the SBT, and the dangers of relying on a short sample period. The sample period for the Treasury data was 1980-81 – a time of deep recession. The SBT system required significant tax liabilities for Michigan firms, even though many were losing money. Given this cyclical intensity of the SBT, the ratio of aggregate SBT Liability to aggregate Business Income would probably drop sharply in good economic times. Without examining the data for a complete economic cycle – down years and good years – an accurate view of the SBT burden is just not available.

The SBT Burden Compared with Profits
The aggregate SBT liability of Michigan firms represented over 20% of their pre-tax profits in the sample period. The SBT clearly intensifies the cyclical pressures faced by Michigan businesses; this ratio probably drops sharply in good economic years.