Final answer:
Belles and Bradlyn conducted a stop smoking study using a cross-sectional design, an observational method effective for finding potential correlations at a single point in time but not for establishing causality.
Step-by-step explanation:
Belles and Bradlyn (1987) conducted a stop smoking study using a cross-sectional design. Cross-sectional studies are a type of observational study that collect data from subjects at one point in time, aiming to find potential correlations between different variables. These studies, like the one on smoking and lung cancer, look for significant associations but do not establish causal relationships due to the lack of control over variables. This method is often chosen for its relative simplicity and cost-effectiveness, but it is generally followed by more in-depth research like case-control or cohort studies to further investigate potential relationships observed.