Claims dependent upon a logical fallacy are generally regarded as not credible. In addition, claims that lack supporting evidence are not given much weight. However, this does not mean that such claims are untrue.
Logic helps form the basis for determining whether a claim is true or not. A claim that violates one of the many logical fallacies is inherently faulty. For example, claiming that a particular remedy is effective because it has been in use for thousands of years violates is an "argument from antiquity," which is a logical fallacy. This does not mean, however, that the remedy does not work.
Some claims are not considered credible because they lack sufficient evidence to support them. After meeting two people from New York named "Steve," someone might claim that all people from New York share that name. However, most of New York's millions of inhabitants are not named "Steve," making the claim not credible.
Some claims are based on anecdotal data. While many people are inclined to believe these claims, they are generally based on small sample sizes. They may be true, but there is insufficient evidence to support them. These claims are still regarded as credible, especially if they are made by a friend or family member.