The results of numerous studies using animals were thus considered fairly definitive. Under these experimental conditions, and as research evidence accumulated, scientists concluded that cigarettes caused cancer and other health problems in animals. Despite these findings, other researchers argued that the link from animals to humans and the resulting conclusion that cigarettes caused cancer in people simply could not be made. The challenge for researchers then became how to study the effects of smoking on human beings. As a result, the research related to humans was correlational in design.
Correlational research allows investigators to explore the relationships between variables.
The stronger the relationship or association between the variables, the more likely a researcher might hypothesize that there is a cause and effect relationship. Association, however, does not necessarily mean causation. Certain individuals discredited the evidence by asserting association does not mean causation. It was not until 1964 that the first Surgeon General's Report on Smoking and Health concluded that cigarette smoking was a cause of lung cancer in men and a suspected cause of death in women. This report was developed after review of more than 7,000 articles that addressed smoking and health outcomes.