An epidemiоlоgist is аnаlyzing dаta frоm a study on lung cancer. The results show that people who smoke '40 cigarettes' per day have a 200% increase in risk of developing lung cancer compared to non-smokers, while those who smoke '20 cigarettes' per day have a 100% increase in risk. What epidemiological principle does this demonstrate, and how does it strengthen the argument for a causal relationship?