We have only correlational evidence that smoking causes lung cancer. Why don't we have cause-and-effect evidence for this relationship? Why do we consider the correlational evidence we do have for the smoking-cancer connection to be strong?

Correlation is a statistical measure that indicates the extent to which two or more variables fluctuate together and can take values between -1 (perfect negative correlation) and +1 (perfect positive correlation). A correlation of 0 means that there is no relationship between the variables. Causation, on the other hand, implies a cause-and-effect relationship between two or more variables.

To establish a cause-effect relationship, one would have to conduct controlled experiments. However, it is not ethical to conduct experiments where participants would be knowingly exposed to a harmful substance like cigarette smoke, for the sake of scientific evidence. This is why we don't have cause-and-effect evidence for the relationship between smoking and lung cancer.

Since we cannot conduct controlled experiments, we rely on observational studies. Observational studies gather data from existing sources (e.g., surveys, medical records, etc.) and look for associations between variables. However, the information gathered from these studies can only establish correlations, not causation. Observational studies are also susceptible to confounding factors, which can make it challenging to definitively establish a cause-and-effect link.

Despite these limitations, the correlational evidence for the smoking-cancer connection is extensive and consistent across numerous studies, indicating a strong positive correlation between smoking and lung cancer. For example, data analysis has shown that smokers are about 15 to 30 times more likely to develop lung cancer than non-smokers. Additionally, the risk of lung cancer decreases significantly as soon as a person quits smoking, further strengthening the correlation.

Beyond correlational evidence, there is also a biological reason to believe that smoking causes lung cancer. Tobacco smoke contains numerous harmful chemicals, including at least 70 known carcinogens (cancer-causing substances). These carcinogens can damage the DNA in lung cells, leading to mutations that can eventually become cancerous. This provides a plausible biological mechanism for the cause-and-effect relationship between smoking and lung cancer. In summary, although we lack direct cause-and-effect evidence due to ethical limitations in conducting experiments, the strong correlational evidence, combined with biological plausibility, convinces researchers and healthcare professionals that smoking has a causative role in the development of lung cancer.