The reason that an appeal to authority is a fallacy is that the deliverer of an argument is irrelevant to the validity of that argument. If an expert makes an

The reason that an appeal to authority is a fallacy is that the deliverer of an argument is irrelevant to the validity of that argument. If an expert makes an argument that is invalid (i.e. doesn't logically demonstrate the conclusion), we may still hold some faith in the argument, because perhaps the expert made some omissions or mistakes in presenting the argument. But we still ought to recognize that the conclusion has not actually been demonstrated as the logical implication of the premises. Obviously the language I'm using in the previous paragraphs applies to formal logic. As with all formal fallacies, they are not so easy to discuss with precision when we're talking about informal arguments (like scientific papers and studies).