A number, usually standardized to have a mean of 100, that can help to identify repetitive patterns. It is typically applied to annual <a href="search.pl?p=time-series data">time-series data</a>, but can also be used for shorter periods, such as hours within a day. (See also <a href="search.pl?p=seasonal index">seasonal index</a>.)<o:p></o:p></P>