In PASSI a sample is extracted from the lists of enrolled residents in each local health units to select a monthly random sample of persons 18-69 years (at least 25 persons per month), stratified by six sex-and-age groups (18-34, 35-49, 50-69 years) based on the proportion of the local population in each of these strata. The sample size allows to obtain annual estimates of the main variables at local health units level with an acceptable precision.
Inclusion criteria include residence in the local health units area and having a telephone number available; exclusion criteria include inability to understand Italian, inability to participate in the interview (for example, those with serious handicaps), and persons who are hospitalized or institutionalized. Letters are sent in advance to those selected explaining the purpose of the system and that they will be contacted shortly; their physicians are also informed.
At least six attempts are made to call on different days of the week (including week-ends) and at different times of day; if they cannot be reached, a substitute from the same sex and age group is randomly selected. This procedure is aimed to ensure the representativeness of the sample at local health units level.
The ongoing surveillance process, with interviews being conducted every month, provides the system with flexibility, because the explored items can be modified with time, and allows the recognition of trends and seasonal variations. Furthermore, by gathering the interviews of two or more years, the sample size can be increased, improving the precision of the estimates.