The correlation between a factor and an outcome could be a coincidence, or it could be caused by a completely different factor.
For example, as ice cream sales increase, sales of meat for barbecues also increase. However, buying ice cream does not cause people to buy barbecue meat. This is influenced by the temperature and weather.
To show a causal link, scientists must find evidence that scientifically explains the connection.
If there is no scientific explanation then there is only a correlation. It cannot be shown that the factor causes the outcome.
There is a correlation between carbon dioxide levels and global average temperatures. What scientific explanation could provide evidence for a causal link between these factors?
The greenhouse effect explains how increasing levels of carbon dioxide in the atmosphere leads to increases in temperature on Earth. This is evidence of a causal link.
Sometimes a change in a factor leads to an outcome but not in all cases. Scientists say that a change in the factor increases the risk of the outcome. This is very common when discussing causes of ill-health.
For example, when nitrogen dioxide levels stay high for several days more people have asthma attacks. However, not everyone has an asthma attack. This type of correlation describes how a factor is connected with an increase in the risk of a particular outcome.
Scientists then try to find a causal link to explain the connection. If no causal link is known, this remains a correlation. It cannot be scientifically shown that the factor causes the outcome.
Data shows that children who live near major roads are more likely to suffer from asthma.
Why is this finding a correlation and not a causal link?
This data shows that living near a major road is associated with an increase in the risk of a child having asthma. This is a type of correlation. Living near a major road does not directly cause asthma, so there is not a causal link between the two.
What research might scientists carry out to try to find a causal link between living near a major road and asthma?
Scientists could research the effects of air pollution on the breathing system of children.