Final answer:
Experimental design is the only research method that can establish a causal relationship between variables due to its ability to control for confounding factors and the intentional manipulation of the independent variable, which is rigorously compared with a control group.
Step-by-step explanation:
The only research design that can truly test causal relationships between variables is the experimental design. The key feature of an experimental design is the experimental manipulation of the independent variable, which is then compared with a control group that does not receive the manipulation. This allows for the establishment of a cause-and-effect relationship because any differences observed between the experimental and control groups can be attributed to the experimental manipulation, assuming the study is properly controlled for other confounding factors.
In an experimental design, the subjects are randomly assigned to different treatment groups, which helps to eliminate lurking variables. By having a control group, the study can control for chance factors. In a well-designed experiment, there may also be blinding and placebos used to maintain the study's integrity. The only difference between the treated and untreated groups should be the experimental manipulation itself, allowing researchers to determine if the independent variable has caused a change in the dependent variable.