Asserting evidence, however, should always convey the speaker's confidence in it. One should be careful to use only evidence that the audience believes can be verified through its own efforts or which has been vetted by knowable and reputable sources. Making sure that evidence meets these criteria for substantiation is important because the criteria create a shared basis for what might be true or not true for both the speaker and the audience. The evidence must be made accessible to the audience and it must be credible to the audience. If the audience finds that the manner in which the evidence presented is consistent with its own understanding of the evidence, then it is likely that the audience will determine that the speaker is knowledgeable and therefore someone whose opinion can be trusted.
Information provided by personal experience performs two roles in a speech.
First, it humanizes the speech by referring to something which many in the audience may have experienced as well. Second, personal experience can only be illustrative, but its implications can be made very clear if the number of individuals or issues involved are limited. The problem with personal experience as evidence, however, is that it is usually not generalizable. There are generally accepted forms of evidence which often require little explanation, but the degree of necessary explanation depends upon the criticality of the evidence. If, for example, one needs to assert something about the size of the American economy in order to make a point about its relation to other economies in the world, evidence from a U.S. Treasury document would need little elaboration or justification.