  Excess mortality is defined as an increase in the number of deaths above what
is expected based on historical trends, hereafter called baseline. In a
previous paper, we introduced a statistical method that allows an unbiased and
robust determination of the baseline to be used for the computation of
excesses. A good determination of the baseline allows us to efficiently
evaluate the excess of casualties that occurred in Italy in the last 12 years
and in particular in the last 3 years due to the Coronavirus Disease 2019
(COVID-19) epidemic. To this extent, we have analyzed the data on mortality in
Italy in the period January 1st 2011 to December 31th 2022, provided by the
Italian National Institute of Statistics (ISTAT). The dataset contains
information on deaths for all possible causes, without specific reference to
any particular one. The data exhibit strikingly evident periodicity in the
number of deaths with pronounced maxima in the winter and minima in the summer,
repeating itself in amplitude along the whole twelve-year sample. Superimposed
on this wave-like structure are often present excesses of casualties, most
likely due to occasional causes of death such as the flu epidemics (in winter)
and heat waves (in summer). The very accurate periodicity along the seasons
(the "baseline"), allows us to determine with great accuracy and confidence the
number of expected deaths for each day of the year in the absence of occasional
contributions. Each of the latter can be modeled with an additional function
that parameterizes the deviation from the baseline.
