The depreciation rates for plant assets are initially determined based on past experience and other relevant information, but these estimates may need to be revised due to unexpected physical deterioration or obsolescence or due to improvements that extend the asset's useful life.

It is also common to need to calculate depreciation for partial years, as assets are often acquired and disposed of at times other than the beginning or end of the accounting period. Factors such as the time of year do not usually play a role in these decisions