Final answer:
A well-designed experiment includes a control group, random assignment helps minimize confounding variables, and the double-blind method involves both the participants and the researchers being unaware.
Step-by-step explanation:
A well-designed experiment includes a control group, which demonstrates what happens when the active treatment is not applied. This helps researchers to compare the effects of the treatment to a baseline. Random assignment is used to minimize confounding variables and ensure that there are no systematic differences between the experimental and control groups. The double-blind method involves both the participants and the researchers being unaware of who is receiving the active treatment(s) and who is receiving the placebo treatment, reducing the potential for bias.
Cross-sectional studies are not useful for establishing causation because they only examine a single point in time, without accounting for changes over time or controlling for other variables.