The Genealogical Proof Standard, step 4

Sources don't always agree. Sometimes someone didn't really know the information they provided, other times someone out-right lied. Which source do you believe when they contain conflicting information?
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 [1]

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
  

Try creating a chart comparing the information you have and the sources it came from. Also consider:

• 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! 

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Sources
[1] The Board for Certification of Genealogist, The BCG Genealogical Standards Manual (Orem, UT: Ancestry Publishing, 2000), 1.