3 Data gathering and processing
3.3 Collecting

In the section on sampling, we presented the different methods to select a subset of the units of a population to carry a sample survey. In this section, we will present the different methods to collect data from this sample. The three important parts of collection are the choice of a collection mode, questionnaire design and the role of interviewers. They are as important in the survey as the sampling plan. First because of their impact on the budget and duration of the survey, but mostly because of their impact on data quality. The right collection strategy should aim to decrease nonresponse as much as possible while a good questionnaire should be developed with the goal of minimizing the measurement error. Nonresponse and measurement error are the two main sources on non-sampling error.