Final answer:
Internal validity is the degree to which a study demonstrates a causal effect of an independent variable on a dependent variable, ensuring the changes observed are due to the manipulation and not external factors. Reliability refers to the consistency and replicability of the study's results. A placebo is used to control for participant expectations, thus safeguarding internal validity.
Step-by-step explanation:
The degree to which a study establishes a causal effect of the independent variable on the dependent variable is known as internal validity. This concept is critical in research design as it pertains to the accuracy of the study, ensuring that the changes in the dependent variable are indeed due to the manipulation of the independent variable and not external factors. In conducting a study, researchers strive to maximize both reliability and validity. Reliability refers to the consistency of a study's results and the likelihood that these results can be replicated in subsequent research. Validity, on the other hand, refers to how well the study measures what it is supposed to measure.
A controlled experiment is a research method where one variable is manipulated and the effects on another variable are observed, establishing a cause-and-effect relationship. Reliability in such experiments is increased by using accurate tools and methods, while a placebo is sometimes administered to control for the effects that participant expectation might have on the experiment, thus contributing to the internal validity of the study.