There are two standard definitions associated with Winter Mortality:
Winter Mortality (WM)
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:
WM =Deaths (Dec to Mar) – [deaths (Aug to Nov) + deaths (Apr to Jul)/2]
Winter Mortality Index (WMI)
The Winter Mortality index is calculated as the number of additional winter deaths divided by the average non-winter deaths expressed as a percentage:
WM Index = (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).
More information on methodology is availble from the associated quality and methodology papers.