To do this we would need the number of deaths which occurred in Sweden during 2010, and the person-years at risk of death lived by the Swedish population in 2010. As Sweden is a country with a complete and reliable vital registration system, obtaining the number of deaths is simple, especially as ‘death’ is a one-off event which is not open to much interpretation! However, calculating the precise person-years at risk is impossible. Therefore we use the mid-year population as an estimate of the person-years at risk.

The mid-year population (MYP) is an approximation for the person-years at risk and is still a widely used measure because calculating person-years for large populations is not easy. MYP is simply calculated as the average of the population at the start of the year and the population at the end of the year. These quantities are easier to find as they can usually be derived from census data.

It is estimated that the mid-year population of Sweden in 2010 was 9 million. MYP is based on the assumption that deaths will be evenly spaced throughout the year, and that on average each person who dies will contribute half a year at risk. This is a fairly safe assumption for large populations of mixed ages. The assumption may be invalid in a few situations. If there are large seasonal fluctuations in mortality rates then deaths will not be spread evenly throughout the year. This assumption is also invalid when considering infant mortality. In developed countries deaths in the first three days of life may account for half or more of all deaths in the first year of life. It is for this reason we use live births as the denominator rather than the mid-year population when calculating the Infant Mortality Rate.