Causal mechanisms
Correlation
Scientists look for patterns in data.
Research has established links between cancer and various Habits which can be modified that impact positively or negatively upon physical and/or mental health., chemicals produced in the body, or that enter the human body, and chemicals in the environment. Scientists have established several causal mechanisms for these Something that increases a person's chances of developing a disease..
Below is an example of how the risk factor of smoking has been studied.
Historically, in the UK, a pattern can be seen between the number of people who smoke cigarettes and the number of smoking related deaths.
As the number of people who smoke cigarettes has decreased over the years, the incidence of smoking related deaths has decreased also. Note that there is a time lag, but cancer usually takes some years to develop.
There is a clear association, called a A relationship between two sets of data, such that when one set changes you would expect the other set to change as well., between the variables.
With cancer and other non-communicable diseases, scientists have found correlations.
Correlation and cause
If there is a correlation between a particular factor and an outcome, it does not mean that the factor necessarily causes the outcome. Scientists must look for a possible mechanism by which the factor could be the likely cause.
In the case of lung cancer, analyses of cigarette smoke have shown that at least 70 of the chemicals present in smoke will cause cancer in laboratory animals which establishes a causal link.