Research designs are classified by the purpose the study is meant to serve, and that classification recognises three of the four names listed.

Exploratory research is used when a problem is still vague and little prior work exists. It is flexible and open-ended — literature scans, expert interviews, focus groups — and its output is a sharpened problem statement or a hypothesis worth testing, not a conclusion.

Descriptive research answers what, who, when, where and how much. It measures and portrays the characteristics of a population or phenomenon as they are, through surveys, observation and case studies, without manipulating any variable.

Causal research, also called explanatory research, answers why. It tests cause-and-effect relationships by manipulating an independent variable and controlling extraneous ones, so its typical form is the experiment.

Subjective research is not one of these design categories. Subjectivity describes a researcher's epistemological stance — the interpretivist position that meaning is constructed rather than merely observed — and it is a property of qualitative approaches, not a separate design alongside exploratory, descriptive and causal.

Hence the name that is not a type of research is subjective research.