What are the desirable properties of the base period?

Base period should have the following properties
(i) The base year should be a normal period and periods in which extraordinary events have occurred should not be taken as base period as they are not appropriate for general comparison.
(ii) Extreme values should not he selected as base period.
(iii) The period should not be too far in the past as comparison with current period cannot be done with such base year as policies, economic and social conditions change with time.
(iv) Base period should he updated periodically.