There are two standard definitions associated with Excess Winter Mortality:
Excess Winter Mortality (EWM)
The method defines the winter period as December to March, and compares the number of deaths that occurred in this winter period with the average number of non-winter deaths occurring in the preceding August to November and the following April to July:
EWM =Deaths (Dec to Mar) – [deaths (Aug to Nov) + deaths (Apr to Jul)/2]
Excess Winter Mortality Index
The Excess Winter Mortality index is calculated as the number of excess winter deaths divided by the average non-winter deaths expressed as a percentage:
EWM Index = (Excess Winter Mortality/Average non-winter deaths)*100
The expected number of deaths is a 4 month average of the number of non-winter deaths: deaths in the two four month periods which precede winter (August to November) and follow winter (April to July).