What is the purpose of defining the 'selected_driver' variable in the cost management process described in the chapter?

The 'selected_driver' variable is defined to specify which cost driver will be modified during sensitivity analysis in the cost management process. This allows the analysis to focus on how changes in that particular driver affect overhead costs while adjusting other correlated variables accordingly.

By defining the 'selected_driver' variable, the analysis can systematically change the value of a specific cost driver, such as 'Labor_Hours', and observe the resulting impact on overhead costs. This targeted approach helps in understanding the sensitivity of costs to variations in that driver, facilitating better decision-making in cost management and optimization.