20 C.F.R. § 404.709
If you give us the type of evidence we have shown as preferred in the following sections of this subpart, we will generally find it is convincing evidence. This means that unless we have information in our records that raises a doubt about the evidence, other evidence of the same fact will not be needed. If preferred evidence is not available, we will consider any other evidence you give us. If this other evidence is several different records or documents which all show the same information, we may decide it is convincing evidence even though it is not preferred evidence. If the other evidence is not convincing by itself, we will ask for additional evidence. If this additional evidence shows the same information, all the evidence considered together may be convincing. When we have convincing evidence of the facts that must be proven or it is clear that the evidence provided does not prove the necessary facts, we will make a formal decision about your benefit rights.
20 C.F.R. §404.709