The two major options of gathering information for a study are personal interviews and questionnaires. They each have their own positive and negative aspects, and knowing what they are allows researchers to use the proper one appropriately.
Questionnaires are a popular method of gathering information for a study. These are a set of lists of questions that are prepared in advance. The papers are handed out or people are shown them on a computer. The point is that they are done by the respondent on their own, without the aid or interference of the surveyor. Doing it this way saves a great deal of time and effort, since several people can be filling out questionnaires at the same time, and one person can distribute dozens of questionnaires. In most cases the questions are multiple choice or short answer, since long, open-ended, or essay questions might lead the respondent to lose interest and not answer honestly, completely, or possibly not at all. This keeps the data simple and of limited scope, but the sample sizes can be larger.
Questionnaires are appropriate for getting categorical data, such as age, gender, income, and such metrics. Multiple choice questions such as “on a scale of one to ten, how do you feel about ______” or “Which of the following cars has the best image: A, B, C, or D”
Personal interviews involve a researcher meeting with a respondent on a one-to-one basis and asking them a series of questions. The researcher has the ability to ask follow up questions either to get more details or to get clarification. The questions can be open-ended and conversational to get whole pictures of how the respondent feels about a subject. The largest down-side to this type of process is that it is extremely time consuming. Either one researcher needs to spend a lot of time talking with people, or the researcher is going to need help. Lacking these options, the interviews would necessarily be short of the quality expected from interviews, or else be a very small sample size – basically defeating the purpose of the study.
Personal interviews are an appropriate method when more information is needed than just a list of data. This is for questions about how people feel about a topic instead of just what they feel. This is for questions that tell a story rather than just a single event. “How have your feelings about _____ political party changed over the last decade?”