What is validity?
Internal validity is the extent to which the study demonstrated a cause-effect relationship between the independent and dependent variables.
External validity is the extent to which one may safely generalize from the sample studied to the defined target population and to other populations.
What is reliability?
Reliability is the extent to which the results of the study are replicable. The research methodology should be described in detail so that the experiment could be repeated with similar results.
Hypothesis - a statement that is believed to be true but has not yet been tested.
Independent variable - the component of an experiment that is controlled by the researcher (for example - a new therapy).
Dependent variable - the component of an experiment that changes, or not, as a result of the independent variable (for example - the existence of a disease).
Bias - prejudice or the lack of neutrality. A systematic deviation from the truth that affects the conclusions and occurs in the process or design of the research.
Confounding - a mixing of the effects within an experiment because the variables have not been sufficiently separated. Possible confounding variables should be discussed in the report of the research.
See also Study Design Terminology from the Levels of Evidence tab.