All studies rely on samples. It is very important that there is no bias in choosing the people in the study. It must be a representative sample so that the conclusions it draws based on the sample can be reliably applied to the whole population.
Matched groups are studies which compare two groups of people who are very similar to each other apart from one factor, for example smoking. Some factors which should be matched are age, sex and weight. Differences in outcomes are therefore more likely to be able to be applied to the whole population.
Scientists look for patterns in data.
Research has established links between cancer and various lifestyle factors, chemicals produced in the body or that enter the human body, and chemicals in the environment.
Results of studies are published in medical journals. This means other scientists can carry out similar studies to see if the results were reproducible - meaning they get similar results. This process is called peer review.
Scientists have established several causal mechanisms for risk factors.
Below is an example of how risk factors have been found. This has come from studies on smoking cigarettes and lung cancer.
Historically, in the USA, a pattern can be seen in the amount of cigarette use and the incidence of lung cancer.
As the number of cigarettes smoked has increased over the years, the incidence of lung cancer has increased also. Note that there is a time lag, but cancer usually takes some years to develop. There is a clear association, called a correlation, between the variables.
With cancer and other non-communicable diseases, scientists have found correlations.
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. This result establishes a causal link.
For mothers who smoke during pregnancy:
The bar chart shows that when mothers under 20 smoke, the birth weight of their babies is reduced.
Suggest two pieces of additional information that would need to be collected before drawing firm conclusions about the effect of mothers smoking on the birth weight of babies.