Sample control

Before the telephone survey starts, the interviewers are provided with lists of phone numbers of potential respondents. These numbers are randomly drawn from the telephone directories and arranged according to the administrative units, inside which the interviewers conduct the survey. Each interviewer receives five times more telephone numbers than he or she is required to do. To ensure the randomness of the sample, there are certain steps the interviewers have to follow when dialing the numbers, e.g. starting from the third phone number on the list, then calling every fifth (step: 3, 8, 13, 18, 23). When they reach the end of the list, they start the next round with the fourth number from the list and then call every fifth (step: 4, 9, 14, 19, 24). If the dialed number is not reaching to the desired respondents, they dial the next number and continue following the given step.
When entering the respondents answers into the data-entry program, the interviewers are obliged to enter each number they dialed. In the next (disqualifying) question they mark one of the answers: – dialed number is not available; – there is no reply, – the respondent refuses to participate in the survey, or some other response, depending on the needs of the specific survey (e.g. if there is a need to make an interview appointment with the respondent). It is possible to go to the next question and continue the survey only if the respondent consents to an interview. Also, if the respondent withdraws from a survey it will be recorded. All these data are exported into CSV, Excel or SPSS file format, as a separate report, or as a part of complete database of the survey.
In case when quotas are given to the interviewers at the start of research, then the completion of these quotas by each interviewer is the subject of an ongoing monitoring and control.

Logical control

Logical control of the interviewers includes standard procedures for verification / authentication of the responses received during the survey:


a) Measuring length of survey. The parameters which can be measured are the duration of entire survey and the length of individual questions, predefined in survey design. This way, during the interview (comparing values ​​obtained from each interviewer) it can be determined whether any of the interviewers filled out the questionnaire more quickly than expected, and whether they “skipped” some questions in the survey. In case that such violation exists, it is possible to intervene quickly and prevent further irregularities.

b) Logical verification of collected data. Comparing and analyzing the responses to specific control questions, in order to determine whether there were significant differences between the responses from different interviewers. If so, the control of the interviewer will be increased.


In case of a field survey with printed questionnaire, the interviewers are required to periodically send completed and signed questionnaires to Plum Mark, via courier-service, after which the control of compliance with the contents of printed survey responses, stored in the entry program, is performed. When all the necessary checks are completed, the quality manager signs and seals 20% of tested questionnaires and, together with other completed questionnaires, if necessary, sends them to the client.

Operational control

Control of the interviewers is made on 20% sample of completed questionnaires of telephone or field surveys and, if some irregularities occur in the interviewer’s work, we perform the control of 100% of questionnaires done by specific interviewer.

Control of the interviewers is carried out on two levels: operational and logical.
Operational (field) control is done by dialing phone numbers of interviewed respondents and a brief conversation with the respondent in order to determine:


1) Whether the respondent participated in the survey,

2) Whether given answers are authentic, which is checked by repeating some predefined control questions from the survey.


In a telephone survey, the interviewers enter respondents’ answers into the data entry software, and after the discussion with the respondent, completed questionnaires are sent directly to the central database, so it is possible to do this kind of control immediately after the interview.

In field surveys, this type of control is possible only when the interviewers enter collected data and send questionnaires, usually several hours after the interview and maximum 24 hours after completing the questionnaire, unless the interviewers use mobile devices instead of printed questionnaires, which allows direct data entry into the program and, therefore, same control as in the telephone survey.