From the Genealogical Proof Standard:
4. We resolve any conflicts caused by items of evidence that contradict each other or are contrary to a proposed (hypothetical) solution to the question 
Compare the conflicting information. You need to decide which source is more reliable. First, what are you trying to prove with your information? A date of birth? The maiden name of a mother? Military service?
Look at the analysis you did in step 3:
- An original source has more weight than a derivative source
- Primary information is more creditable than secondary information
- Who gave the information?
- Does the grandson-in-law really know the birth information for a death certificate?
- What is the record?
- Are you looking at the marriage affidavit, license, certificate or return?
- Why was the record created?
- E.g. on a marriage affidavit someone has sworn that both parties are of legal age.
- When was the record created?
- A delayed birth return might be less creditable than a return made a day or two after the birth.
Always keep in mind...
Sources can lie!________________
 The Board for Certification of Genealogist, The BCG Genealogical Standards Manual (Orem, UT: Ancestry Publishing, 2000), 1.