For assembly cost pools, the number of parts is typically the most accurate cost driver, directly correlating with the required assembly time or machine use.
The best cost driver for the assembling cost pool would be the number of parts. This is because the assembly costs are most directly affected by the number of parts being assembled into a product. Each additional part typically requires additional assembly time or machine use, making it a more accurate reflection of the assembly cost than the number of product lines, the number of orders, or the amount of square footage. The latter might influence other cost pools but are less directly related to the intricacies of the assembly process itself.
So, the number of parts is the most practical and relevant cost driver for determining assembly costs, providing the most accurate allocation base for cost accounting.