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My Great Grandfather is in the 1900/1910/1930 census, BUT he is not in the 1920 census. Why?  And where do I go?
Sometimes a family just may not be indexed but it does not mean they are not there.  Keep in mind, as you have either the first page that pulls up the printed text version of the family, and to your left is a block is the actual view version of actual census page.  List several names of neighbors and place that to search in the search area.  Pull up census page of his neighbors and read each entry carefully.  You may locate your Great Grandfather on a page of the original census.  His name and the family just did not get indexed.  Cross check due to his age and a possible widow, he may be living with one of his children.  Cross check against a City Directory.  So many have been posted at Ancestry.com; or call local library for his residence (reference department) and ask them to do a 'look up' in the 1920 city directory. 
Why is a death certificate not considered a Primary Record?
Only portions of the death certificate are considered primary.  The deceased's name, the certificate number at the top right of the certificate, the time of death, when the certificate was filed, the cause of death, location of death, and the signature of the attending Doctor.  Locate the name of the informant on the left side, lower section.  This name could be a neighbor, a great grandson, or other individual who would have no really correct family information such as his dead wife's maiden name or the names of the deceased parents. This information, then, would be secondary and subject to verification.