The only method that can establish a cause and effect relationship is the experimental method. In this approach, researchers manipulate one variable (the independent variable) to observe the effect it has on another variable (the dependent variable).
By controlling for other variables and using random assignment, researchers can ensure that any changes in the dependent variable are actually due to the manipulation of the independent variable, rather than other factors. This level of control is what distinguishes the experimental method from other research methods, such as observational studies or surveys, where it can be difficult to determine whether a relationship is truly causal.
For example, in a clinical trial testing a new medication, researchers can randomly assign participants to receive either the medication or a placebo. By comparing outcomes between the two groups, they can establish whether any observed effects are a direct result of the medication rather than other influences.