If a practice or belief is justified there must be good reasons for it and those reasons should explain why the practice is a good one or why the authoritative person or text supports it. The irrelevant appeal to authority differs from the appeal to an irrelevant authority.
An example of an irrelevant appeal to authority would be to claim that vaccines are not safe because Dr. Jay Gordon, a pediatrician and assistant professor of pediatrics at UCLA Medical School, says they're not. Quoting Gordon's reasons does not make the appeal to his belief relevant to whether vaccines are safe. The following claims don't become true just because Dr. Gordon asserts them.
Studies showing that vaccines and their many constituents do not contribute to this problem [of triggering autism] are flawed, filled with specious reasoning and, for the most part funded by the pharmaceutical industry. Even articles in reputable medical journals are often written by doctors with an economic interest in continuing the vaccination program's status quo. This does not invalidate all of these studies but it certainly makes them suspect and a poor foundation for an argument excluding vaccines from the list of environmental influences on the increase in autism in America and elsewhere.Since there could be nothing more relevant than scientific studies to the issue of whether vaccines trigger autism, it begs the question to dismiss scientific studies as "suspect." To cite Dr. Gordon in support of not considering scientific studies when trying to determine whether vaccines trigger autism is irrelevant. A proper approach would be to analyze and evaluate the studies that defenders of the safety of vaccines put forth as the best ones showing there is no association between vaccines and autism. That is the approach Dr. Gordon should take and it is the approach anyone citing him to support the belief that vaccines aren't safe should take. Dr. Gordon may be an expert in medicine, but the value of the studies on the association between vaccines and autism depends on the nature of those studies, not on his say-so. In any case, there are many other experts, just as qualified as Dr. Gordon, who disagree with him. The fact that Gordon and other experts disagree with each does not make the issue controversial, however. Gordon is out of step with the consensus of medical experts that vaccines are safe and not associated with autism. Finding an outlier who disagrees with the scientific consensus does not mean you've established that there is a controversy over an issue. Some in the mass media present outliers in a feeble attempt at fairness (pseudosymmetry). To be controversial, there must be widespread disagreement among the experts about the issue.
(As an aside, I have looked at the scientific studies and my opinion is that there is no compelling evidence of an association of vaccines with autism or that pharmaceutical firms have corrupted the research process in this area. Don't take my word for it, though. Read what I have to say about the studies and then check them out for yourself.)
An example of the appeal to an irrelevant authority would be appealing to the advice of an actress with no education or background in medicine to justify seeking some offbeat cancer treatment or for claiming that common vaccines would be harmful to children. Citing Jenny McCarthy on scientific or medical issues is to cite an irrelevant authority. Being a mother of a child who you declare is autistic does not make you an instant expert, no matter how many conversations you've had with supporters like Dr. Jay Gordon.
It’s often the case that arguers combine the irrelevant appeal to authority with the irrelevant appeal to popularity. If it is irrelevant to appeal to one authority to prove a point, then it is irrelevant to appeal to many authorities to prove the same point. However, it is not always irrelevant to appeal to authorities. If you know nothing about medicine and your physician goes over the results of a medical test with you and recommends a course of action, you are not committing the fallacy of irrelevant appeal to authority when you justify taking that action because your physician recommends it. You might consult another physician for a second opinion, but you would be foolish to consult, say, the janitor, Suzanne Somers, or the local newspaper’s astrologer.
We must rely on experts sometimes, but experts don’t always agree with each other. If, for example, your medical test involved some back problems you’ve been having, you might get five different opinions from five equally competent physicians on what course of action would be best for you. Why? Recommendations for back problems are notoriously controversial. It would obviously be silly to claim that one recommendation must be the best one since it was made by an expert when there are five different recommendations from five equally competent experts. Ultimately, you should consider all the pros and cons of each of the recommendations and select the option that seems best to you. On the other hand, if four of five equally competent physicians recommend the same course of action, unless you can find a compelling reason for rejecting their position, it would seem that the reasonable course of action would be to follow their advice.
When the majority of experts in a field agree on something, we say there is a consensus. Such is the case with climate experts on the issue of anthropogenic global warming. There are many people, some of them scientists, who do not agree with the consensus that human activities such as deforestation and burning of fossil fuels that result in more greenhouse gases like carbon dioxide are causing changes in our planet’s climate that may prove devastating and irreversible. One tactic of the climate change deniers is The Petition Project, which features over 31,000 scientists signing a petition stating “there is no convincing scientific evidence that human release of carbon dioxide will, in the foreseeable future, cause catastrophic heating of the Earth’s atmosphere.” It is true that 31,000 scientists is a large number, but it is irrelevant to the issue of whether humans are largely responsible for climate change. Most of these 31,000 scientists aren’t experts in climate science and, in this case, that matters because when anyone speaks outside his or her own area of expertise their view carries no more weight than that of any other non-expert. What makes it reasonable to accept anthropogenic climate change is not the fact that almost all climate scientists agree. It’s why they agree. Even non-experts can figure out that the experts agree: a survey of all peer-reviewed abstracts on the subject ‘global climate change’ published between 1993 and 2003 showed that not a single paper rejected the position that global warming is largely caused by human behavior. Climate scientists are not arguing about whether global warming is happening. They’re not arguing about whether humans are largely responsible for global warming. They may be arguing about what action to take. In that case, they should be considered as advisers by those who make policy. Unfortunately, many of those who make policy seem to be ignoring the climate scientists in favor of beliefs pushed by gas, oil, and other corporate interests. Those interests should be considered, but not to the exclusion of the science experts.
A similar tactic has been taken by a group of religious scientists who believe that the scientific fact of evolution is inconsistent with their interpretation of the Bible. The Discovery Institute ran an advertisement in 2001 with the heading “A Scientific Dissent from Darwinism.” (“Darwinism” is not a scientific term but a polemical one used by anti-evolutionists to mean evolution.) The ad had over 700 signatures of “doctoral-level scientists and engineers” and stated that “Darwinism,” i.e., evolution, isn’t sufficient to account for the complexity of life. According to the Discovery Institute, only an intelligent designer, viz., a god, could account for the complexity of life. Even if that were true, appealing to a group of scientists and engineers who agree with it isn’t relevant to the claim’s truth.
(As an aside, I find it interesting that The National Center for Science Education has recently expanded its mission to include climate change education in addition to evolution education. The tactic of trying to get creationism taught in public schools under the guise of fairness and "teach the controversy" [created by the creationists!] is now being used by climate change deniers. For an example, see the bill recently introduced into the Oklahoma state legislature.)
Relevance should not be confused with significance or sufficiency. It’s relevant to bring up the fact that shortly after a vaccination your child was diagnosed with autism. Of course, having your child diagnosed with autism is significant to you. But in terms of evidence, the many scientific studies that have found no association between vaccinations and autism carry more weight than a single personal experience of one thing happening after another (see the post hoc fallacy). And your intuition that a vaccine caused your child's autism is clearly not sufficient evidence to justify that claim, no matter how confident you are of the connection.