As every critical thinker knows, anecdotes are the main ammunition for pseudo-science. Since they have no hard evidence to back up their claims, one is usually presented with the old argument "I've tried it; it works". But is all anecdotal evidence fallacious?
Your Rockstar says no. Anecdotal evidence is typically unreliable, but not guaranteed to be false.
Part I or "Einstein > Moron"
Prior to modern medicine, many cures were determined solely on anecdotal evidence. For instance, James Lind observed that the 18th century British Royal Navy had fewer cases of scurvy when the sailors ate fresh citrus fruits. One of the common methods of preventing scurvy was sucking on limes - which is supposedly where the American slang term for Brits, "limey", came about. So it became anecdotal evidence in the form of "we're not sure why, but sucking on limes prevents scurvy".
Why was this an example of good anecdotal evidence? Due to the tales, Lind decided to make observations based on his knowledge of science and nutrition. Lind figured there was a plausible explanation. He made predictions, and was able to test his hypothesis, making the cure for scurvy in the 1700's akin to Einstein's idea of a "cosmic force" (one recently shown to be plausible due to the fact the universe is not only expanding faster than expected, but gaining speed. I'd probably need the Bad Astronomer to confirm that). Unfortunately, like Einstein, the theory could not be proven until later in time when Albert Szent-Gyorgyi discovered the properties of Vitamin C.
Part II or "Dealing with Fools"
Bad anecdotal evidence is that which deals with the paranormal, supernatural, magic, or that which has already been proven false. For instance, not only is there no evidence for astrology, but it's been proven that astrology doesn't work at all. The same can be said for the anti-vaccination nutbars. Yet there are still assholes out there using anecdotal evidence to try and "prove" this silliness to the rest of the world.
Take Phiten® Titanium products. Via Skeptico we learn magic necklace salesman Scott McDonald states:
Everybody has electricity running through their bodies… This product stabilizes that flow of electricity if you're stressed or tired. Pitchers are seeing that they aren't as sore. Injured players are seeing that they recover faster from workouts. People are always skeptical, but when they try it, they become believers.
Translation: Our necklace is just a piece of metal. So we'll come up with some scientific words and anecdotes to correlate our magic necklace to stress relief!
Skeptico further reports:
Phiten® Titanium products promote pain relief and performance enhancement. Titanium emits energy that is effective in controlling the flow of bioelectric current. When this current is stabilized, the muscles relax and blood circulation increases, allowing for easier movement and pain relief. All Phiten® products help relieve pain in muscles and joints by improving the alignment of ions, especially at the body's crucial motor points. Our product is also effective in the temporary relief of pain from bursitis, sciatica, tendonitis, certain types of arthritis and numberous other conditions.
Riiiiiggghht. Titanium emits "energy"? What energy? (Ad Hominem alert) "Numberous"? Very respecatabel...
They go on to provide gleaming testimonials from baseball players and the like, soundly endorsing the product.
Part III or "But Rockstar, why is this bad anecdotal evidence and Lind's limes aren't?"
Lind heard the anecdotes - "Scurvy-grass cures and prevents scurvy!" Using his knowledge of nutrition, it sounded plausible. He made observations, created a hypothesis, and construed and tested a valid theory. This theory was later proven to be scientific fact.
Metallurgy tells us there is absolutely no reason to believe Titanium emits "energy". There is no reason to believe the "stabilizing the bioelectric current" would assist in curing dick. The only people to observe the effects of the magic necklace were those using the product. The easiest person to deceive is one's self.
Anecdotal evidence can be useful in the realm of science. The main difference between good and bad:
Good anecdotes lead to scientific discovery.
Bad anecdotes are used to prove something against all observation and logic.