Quantitative research produces data that is expressed numerically, such as test scores or measures of length or width.
Qualitative research "aims to describe and explain individuals' subjective experiences, actions, interactions, and social contexts through various approaches"
Mixed methods research combines both quantitative and qualitative research method, realizing the strengths and limitations of each.
Experimental research The basic characteristic of all experimental research is that the investigator manipulates an independent variable (the variable that is expected to produce an effect) in order to affect a dependent variable (the variable in which a specific outcome is observed). In experimental research, participants with similar characteristics are randomly assigned to either an experimental group (exposed to independent variable) or a control group (not exposed to independent variable) and researchers compare the outcomes of the groups.
Quasi-experimental research is similar to experimental research but either lacks a control group or participants are not assigned to groups at random.
Observational research uses non-experimental methods to generate quantitative data about participants' behavior within a specific environment.
Correlational research is a type of observational research that examines the relationships between variables, but unlike experimental or quasi-experimental studies, correlational studies lack active manipulation of the independent variable(s). Correlational studies cannot, by themselves, establish a cause-and-effect relationship between variables, but are helpful in establishing that a relationship between variables warrants further investigation.