Final answer:
While controlled experiments allow for cause-and-effect conclusions by controlling variables and manipulating the independent variable, observational studies cannot prove causation due to lack of control and potential confounding variables. Observational studies are still valuable when experiments are unfeasible and can offer larger data sets.
Step-by-step explanation:
Controlled experiments are essential for establishing a cause-and-effect relationship because they involve manipulation of an independent variable and control over extraneous factors. This design allows researchers to isolate the effect of the independent variable on the dependent variable. In contrast, observational studies do not involve manipulation or controlled conditions, meaning they can't conclusively prove that one thing causes another, as they may be affected by confounding variables.
However, observational studies are important and are undertaken when controlled experiments are not feasible, such as when they are impossible, impractical, or unethical. Moreover, observational studies may be more cost-effective, can potentially involve a larger number of subjects, and are better for gathering broader data sets. For example, studying the long-term effects of smoking on health would require an unethical experimental design, thus observational studies become the primary tool.
The ability to assert causation is the greatest strength of experimental research. Experiments can make causal statements because they control for biases and expectancy effects, ensuring that significant differences in the findings are solely due to the independent variable. Therefore, if a study finds that one group's behavior differs significantly from another's under controlled conditions, researchers can safely make a causal claim.