Interviews, polls, surveys, telephone research (CATI)

Sampling

We define universe as a population or enterprise that has information that must be collected by means of a questionnaire.

Once we define the universe, we define the sampling method used for extracting a representative sample of the population: i.e. one that is able to supply us with an estimate about behaviors or opinions.

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Sampling can be done in various ways:

List: we extract a random sample from a list of phone numbers, inserting them into our software's database. Demetra owns phone number lists of the Italian population and Italian companies.

Using our software, we extract a random proportional sample, eg from areas focused on in a questionnaire.

Quotas: we establish a number of units interviewed, characterized by a specific attribute (for example n male and m female). The contact phase continues until the established quotas are covered. Typically, quotas respect the universe’s composition.

Random Digit Dialing: once the first digits of a phone number are established, a random procedure generates the remaining digits; when a valid phone number is found, the procedure creates the other numbers with digits either preceding and/or following. With this method, the family contacted becomes the sampling unit.

From the family to the person: if the sampling unit is represented by people, a random selection method is necessary for determining a single component of the family:

- if there is only one component, he/she will be interviewed;

- once we know the number of males and females, a specific algorithm will choose one element of the family;

- Last birthday method: we will interview the person whose birthday is nearest the day of the interview.

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