Tonight I read yet another post from an anti-vaccine advocate citing epidemiologists who can prove a link between vaccination and autism. Those must not be very ethical epidemiologists. Science is seldom certain of anything. It is rare that a researcher can say X is definitely, undoubtedly, always caused by Y.
Only in public policy debates do we make science seem certain of things. In academic papers, everything is reduced to correlation and likelihoods, seldom are there certainties. Being certain can lead to embarrassment later in most fields.
Epidemiologists are professionally constrained from stating that "X causes Y." They can offer correlation, probability, and other measures of statistical significance, but they are not experts in causation.
In fact, the U.S. Code prohibits the use of epidemiology as the sole or even primary evidence in a case relating to causation. The British and Canadian courts have similar restrictions on epidemiologists.
An epidemiologic study published in a medical or scientific journal, is legally "hearsay." An epidemiologic study that is performed by the government, or funded by and subject to oversight bodies, such as one performed by/for the Centers for Disease Control (CDC), may be admissible based on the hearsay exception for government records contained in Federal Rules of Evidence 803(8)(c).
Epidemiology is, at least legally, only evidentiary in rare instances. Usually, authors of university studies are not permitted to testify as paid experts (COI regulations), and the CDC/NIH have even more restrictive policies.
Quoting from the Federal Judicial Center (fjc.gov):
"A final caveat is that employing the results of group-based studies of risk to make a causal determination for an individual plaintiff is beyond the limits of epidemiology."
Epidemiology is, at best, able to recommend what other fields study and test. That's extremely important, undoubtedly, but epidemiologists are not research scientists — they are statistical experts from fields such as bioinfomatics and empirical sociology.
I doubt the explanation matters, though. I have heard "epidemiologists" working for private groups make claims beyond what the professional organization allows.
The Association for Professionals in Infection Control and Epidemiology is the main body, followed by the American College of Epidemiology. APIC doesn't even include the "E" in their official logo, though — epidemiology is not viewed as existing within the medical field, but as an advisory field.
Epidemiologists have no state boards, no licensing requirement, and are not regulated as medical professionals would be. You can call yourself an "epidemiologist" with any degree specializing in statistical analysis of infection and/or disease.
Seriously, these are the ACE requirements:
Doctoral degree, or doctoral equivalent, as follows: In epidemiology or in any field relevant to epidemiology, with specific formal training in epidemiology of at least one year's duration.
I admire many epidemiologists, including one I worked with as an undergraduate during the start of the AIDS crisis in the 1980s, but I also know the risk of including epidemiological data in a paper. Such data are almost always challenged as "not also considering factors X, Y, and Z." Correlation is not causation. Not in science.
Epidemiology is a guide. It is a needed and important empirical field that helps researchers. At its best, epidemiology can help locate where and how events started. It can also exclude some causes, statistically, but it can never answer with perfect certainty what caused X infection or disease in a population.