According to Nguyen Hai Shui (2013). Staff to meet directly the object of the investigation to interview according to a questionnaire composed available. Applied research of complex phenomena, the need to collect more data; When want to poll the audience through brief questions and can answer fast, ... Due to face-to-face respondents so staff could convince them to answer, can explain to them about the question, the image can be used in combination with the words to interpret, can examine the data in place before record survey. However the high costs, lost a lot of time and effort.
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