The number of deaths in a given year as a percentage of those expected (expected = standard mortality of the reference period, adjusted for age and sex).
The ratio of the number of deaths occurring to residents of a geographic area to the expected number of deaths in that area based on age-specific mortality rates.
Is a good measure for comparing mortality data that are based on a small number of cases or for readily comparing mortality data by geographical area.
World Health Organization. European Observatory on Health Systems and Policies. Glossary. 2009.