Quantitative research is used to generate numerical data or data that can be converted into numbers. Study types that are used in the health field include:  

• Case report or case series - a report on one or more individual patients.  There is no "control group" so this study type is considered to have low statistical validity
• Case control study - looks at patients with a particular outcome (cases) as well as "control patients" who don't have that outcome. Is useful in aetiology (causation) research but prone to causation error
• Cohort study – identifies and follows two groups (cohorts) of patients, one having received the intervention being studied, and one having not. Useful in both aetiology and prognosis research. Because the groups are not randomised, they may differ in ways other than in the variable being studied
• Randomised Controlled Trial (RCT) - a clinical trial in which participants are randomly allocated to a test treatment and a control. This is considered the “gold standard” in testing the efficacy of an intervention. RCTs incorporate the techniques of randomisation and blinding, which reduce the potential for bias and provide good evidence for cause and effect.

Qualitative research is used to explore and understand people's beliefs, experiences, attitudes, behaviour and interactions. It generates descriptive, non-numerical data.  Qualitative research methods include:

• Documents - the study of documentary accounts of events, such as minutes of meetings
• Passive observation - the systematic watching and recording of behaviour  
• Participant observation – here, the researcher occupies a role or part in the setting, in addition to observing
• In-depth interview - a face-to-face conversation to explore issues or topics in detail
• Focus group - a method of group interview which explicitly includes and makes use of the group interaction to generate data.

A research study does not have to be exclusively quantitative or qualitative. Many studies will use a combination of both types of research.

In the Dictionary of Statistics and Methodology, Mixed-Method Research is defined as:

"Inquiry that combines two or more methods. This particular term usually refers to mixing that crosses the quantitative-qualitative boundary. However, that boundary is not necessarily the most difficult one to cross. For example, mixing surveys and experiments (both quantitative methods) may require more effort for many researchers than combining surveys and focus groups (the first quantitative and the second qualitative)."

Mixed method research. (2005). In P. W. Vogt (Ed.), Dictionary of statistics & methodology (3rd ed.). http://dx.doi.org.ezproxy.csu.edu.au/10.4135/9781412983907.n1190