Correlation versus causation is a distinction many people in the general public do not understand. A correlation signifies that as one variable, such as a harmful substance, is increased another variable, such as prevalence of disease, also increases. Causation for public health matters is extremely difficult to claim, especially in humans. To claim causation, requires proof that subjects were exposed to a harmful substance before a disease was developed.
In terms of the impacts of the unconventional gas development industry, this temporality is difficult to gather evidence on. Studies can not intentionally expose humans to fracking, particulate matter, or contaminated substances, because it would be unethical to continue until subjects develop cancer or other health-related issues. If a study were to be conducted outside of a laboratory, it would be near impossible to control all extraneous variables. This is why there is little research proving that unconventional gas development causes adverse health impacts. This clarification is very rarely, if ever covered in the media, which is why many in the general public do not understand this concept. Misunderstanding within the general public is often exploited by proponents of the unconventional gas development industry.
We at PSR work every day to clarify this distinction. PSR has published a compendium of over 1,700 entires, with more than 80% providing evidence for a correlation between fracking and "signs of harm or indication of potential harm" on humans.
As stated beautifully by PSRPA's Medical Advocacy Director, Laura Dagley, "Causation is really hard to prove... But correlation is the first red flag of public health... Once you see causation, that usually means the problem has been around for too long".
CC BY-NC 2.0, by Jyrinx, "XKCD: Correlation", www.flickr.com/
Or, if you prefer Dilbert: https://dilbert.com/strip/2011-11-28