After they have completed the instrument, ask them if there were any questions they did not understand. Respondents in an effort to avoid looking stupid may fill out a question even if they do not understand it. Also check to see if the response categories are appropriate. Sometimes you will need to add another category or revise the wording of possible responses. The more the better is not true of sample size.
Quality is more important than size.
The goal of sampling is to obtain responses from representatives of the entire population of interest. Simply interviewing friends or those who only shop on Saturdays will probably not result in reliable data. Those not interviewed may differ from those interviewed. Consult a marketing research text for more information on sampling designs. Data Collection Data collection typically involves supervising those collecting the data.