a) the event does not exist;
b) the event exists and is measured, but no cases have been recorded.
Four dots ( .... )
the event exists, but all data related to the reference year are unknown for whatever reason, causing an interruption in the time series.
Two dots ( .. )
a) values are less than half of the minimum unit used;
b) the values calculated are not significant due to the small dimension of the event.
The value is omitted to safeguard statistical confidentiality.
Three plus symbols (+++)
for variation higher than 999.9 percent.