seat-belt usage (B) has no causal effect on smoking (X) or lung disease (Y); it is merely an indicator of a person’s attitudes toward societal norms (A) as well as safety and health-related measures (C). Some of these attitudes may affect susceptibility to lung disease (Y). In practice, seatbelt usage was found to be correlated with both X and Y; indeed, in a study conducted in 2006 as part of a tobacco litigation, seat-belt usage was listed as one of the first variables to be controlled for. If you accept the above model, then controlling for B alone would be a mistake.
