There is usually more than one source for family information - like a person's birth date. It is important to remember that different sources might have different origins for the same information.
For example, a Draft Registration Card will usually indicate the birth date that was provided by the registrant about himself. But a Death Certificate will usually indicate the birth date provided by a spouse, child or other relative of the deceased.
Here is a list of the major sources of family information - click any one for a brief discussion of that information source and where that information might be found.