An informal fallacy occurs when the contents of an argument’s stated premises fail to adequately support its proposed conclusion. Fallacies of this type are the “types of mistakes in reasoning that arise from the mishandling of the content of the propositions constituting the argument“. In contrast to a formal fallacy of deduction, the error is not a flaw in the form of the argument. Though the form of the argument may be relevant, it is also the content that is implicated in the erroneous reasoning. So while formal fallacies always guarantee that the resulting argument is invalid, an argument containing an informal fallacy might employ a valid logical form while nevertheless remaining rationally unpersuasive.
A special type of informal fallacy is the set of inductive fallacies. Here the most important issue concerns inductive strength or methodology (for example, statistical inference). In the absence of sufficient evidence, drawing conclusions based on induction is unwarranted and fallacious. With the backing of empirical evidence, however, the conclusions may become warranted and convincing (at which point the arguments are no longer considered fallacious).
For instance, the informal fallacy of hasty generalization can be roughly stated as an invalid syllogism. Hasty generalization often follows a pattern such as:
X is true for A.
X is true for B.
Therefore, X is true for C, D, etc.
While never a valid logical deduction, if such an inference can be made on statistical grounds, it may nonetheless be convincing. This is because with enough empirical evidence, the generalization is no longer a hasty one.