The ad hominem fallacy has nothing to do with trying to undermine the credibility of a witness by providing evidence of his untrustworthiness. Testifying is not arguing. It is reasonable and relevant to question the motives or character of someone who is testifying. Testimony stands or falls on whether the claims made are believable. Jurors may draw conclusions based on testimony, but the one testifying is making claims not arguments. The ad hominem fallacy occurs only when one attempts to refute another person’s argument by focusing on the arguer rather than the argument.
A favorite ad hominem of those who do not like the arguments of defenders of scientific medicine against the use of such practices as distant healing, homeopathy, or acupuncture is to claim that those who practice scientific medicine oppose alternative medicine because it cuts into their profits. The same fallacious appeal is often made by opponents of so-called alternative health practices. Presumably, anyone who makes a living from providing health care expects to profit from it. So what?
A favorite form of poisoning the well is to sprinkle value judgments throughout one's rebuttal. For example: "I can't believe they let you teach critical thinking. Your standards are so low that you are a danger to your students."
“Carroll is a committed skeptic who is strongly motivated to try and discredit the positively and statistically significant results of these tests, which imply some form of unexplained communication between Aimee and N’kisi.”
Carroll is “a committed ideologist who wants to censor what the public gets to know.”
“Of course he is free to dismiss with contempt any research he doesn’t like, but he is wrong to mislead his readers by pretending to be scientific.”
“Carroll’s comment is deliberately misleading.”
“Carroll has no scientific credentials, and he gets carried away by his strong beliefs and dogmatic zeal. His style of analysis is amateur and pretentious; his intentions are polemical.”