The type of study that allows us to draw firm conclusions about the cause-and-effect relationship between two variables is an experimental design. In an experimental design, researchers manipulate an independent variable and measure its effect on a dependent variable while controlling for other potential variables. This allows for the establishment of causal relationships between variables. Correlational studies can provide information about the relationship between variables, but they do not establish causation. Quasi-experimental designs may approximate experimental designs, but they often lack the random assignment of participants to conditions, which is a key feature of true experiments.