Because it is inextricably intertwined with financial reporting quality, audit quality also depends on firms' innate
characteristics and financial reporting systems. Therefore, it is critically important for models that empirically test audit
quality to disentangle these constructs. In this section we illustrate how some of the commonly used audit quality models
control for firms' innate characteristics and financial reporting systems. Table 3 reports four models commonly used in tests
of the following audit quality proxies: GCs, DAC, audit fees, and Big N. The top row of Table 3 lists the control variables that
are typically used in these models and the second row reports some of the studies that use these models. We emphasize,
however, that these control variables are provided for illustrative purposes only. Most studies include additional control
variables, because each treatment variable has its own set of potentially omitted correlated variables. While the control
variables in Table 3 may provide a starting point, they are by no means comprehensive.