While preparing the financial statements, many amounts due to uncertainties inherent in business activities are estimated.  For example, the useful lives of property, plant, and equipment, fair values of financial assets or financial liabilities, bad debts, inventory obsolescence, provisions for warranty obligations.  Accounting estimates change usually as circumstances change or with better estimates being made available. Thus, a change in estimate does not warrant restating the financial statements of a prior period because it is not a correction of an error.

Accounting and Disclosures for the changes in the estimates:  Changes in accounting estimates are to be adjusted prospectively in the period in which the estimate is amended.  An entity should also disclose amounts and nature of changes in accounting estimates, with disclosure of changes relating to future periods, unless impracticable.  

Occasionally, it may be difficult to distinguish between changes in measurement bases (i.e., the accounting policies) and changes in estimate.  In such cases, the change is treated as a change in estimate.

(References: IAS 8, Wiley IFRS: Practical Implementation Guide and Workbook)