Age standardization of data was performed to
account for changes in the age composition of
the population during the study period. The
principle is to apply the age-specific incidence
rates from each year of our study population to the
age distribution of a standard population. WHO
recommends basing the standardization on the
average age composition of the populations compared
in the period (27, 28). Data on the age
composition of the Danish population are available
through Statistics Denmark.