All research conducted in the social sciences, and in fact most research in all areas of inquiry, is often divided into qualitative and quantitative. Quantitative research gathers data that can be numerically coded and uses mathematical methods of analysis. Qualitative research gathers in-depth, individual data to gain insight, describe fully and understand their subjects.
The distinction between qualitative and quantitative research is by no means absolute and it's better to think about qualitative and quantitative data. However, different types of research lend themselves better to gathering either qualitative or quantitative data.
Typical quantitative research types include experiments, quasi-experiments and correlation studies. The main types of qualitative research are observation and interviews. Case studies use both types of data, but are essentially qualitative in character.
Experiments and Quasi-experiments
The experimental design of a research project allows the scientist to manipulate the independent variables and conduct cause-effect analysis. Researchers assign subjects to experimental groups at random and use control groups or dual measurements to control the effect and its direction. Quasi-experiments have no random assignment to groups, but the researchers use control groups or multiple measurements. Most if not all experimental studies yield quantitative data in the form of numbers that can be counted and measured.
In many cases, it's not possible to randomly assign subjects to experimental groups nor use control groups. The phenomena are out there in the world; all the researchers can do is to measure them and see what other variables they coincide (correlate) with. Correlation studies in social sciences often use survey methodology, where a large samples of respondents answer a set of standardised, closed questions.
Observation typically produces qualitative data, and is commonly used in sociology and for projects applying the ethnographic approach. Participant observation takes place in the natural environment, usually a specific community or location. Researchers use field notes and sometimes audio or video to record their observations. Observation can yield quantitative data, too; for example, when studies count instances of behaviour at specific time intervals.
Interviews can yield both qualitative and quantitative data. Standardised surveys on large samples of respondents provide large amounts of quantitative data that researchers analyse using statistical techniques. On the other hand, in-depth individual interviews and focus groups generate detailed, individual data that is then analysed and interpreted in a qualitative way.
Case studies attempt a comprehensive, deep understanding of a specific individual, experience, organisation or community. Researchers preparing case studies collect both qualitative and quantitative data using a variety of techniques and sources. Essentially, though, case studies are qualitative as they allow for neither controlled manipulation in experimental groups nor representative picture of the population.