Quantitative and qualitative research are two approaches to gathering data. Although they can be used together, and the two approaches get into similar methodology at times, they look for different types of information. Each approach has advantages and disadvantages, but what works best depends on what you're studying, how much information you need, and how much detail you need.
Quantitative research methods code data to numerical values so a lot of data can be gathered efficiently. For example, researchers looking for quantitative information could ask survey questions like, "On a scale of 1 to 10, how happy are you in your relationship?" This question is quantitative because it asks the person taking the survey to assign a numerical value to what the researcher is studying (happiness in relationships). Surveys and interviews are the primary methods of gathering quantitative data.
Qualitative research takes longer to conduct, because the data researchers gather is more open-ended, and may not relate directly to a numerical value. For example, researchers looking for qualitative data would ask a question like, "How do you feel about your current relationship?" This question is qualitative because the answer can be nearly anything, and doesn't have to reference a numerical scale that the researcher created. Observation, surveys, and interviews are the usual methods for gathering qualitative data.
Quantitative vs. Qualitative
Although researchers ask similar questions and use similar data collection methods in both quantitative and qualitative research, the important difference is the type of data the researcher is looking for. Quantitative research is quicker and cheaper to conduct, but it doesn't provide the in-depth answers that qualitative research does. It's also more difficult to gather qualitative data because people don't always spend the time required to answer qualitative questions. Researchers can mix quantitative and qualitative research methods to gather a lot of information efficiently.
When doing any type of research, it's important to know what type of information you need the most. Whether you need a lot of detailed information (qualitative) or a lot of general information (quantitative) about your subject will guide how you construct your study, from the way you word survey questions to how much time and money you'll need to spend on the research.