The Social Security Death Index (SSDI), an often-used source of information, is more formally known as the index to the Social Security Administration Death Master File. The file is created from Social Security Administration (SSA) payment records and includes for each decedent (if the information is available to the SSA): name, Social Security number (SSN), date of birth, date of death, state where the SSN was issued, ZIP code of the last residence, and ZIP code of where the death benefit was sent.
Because the SSA records were not automated until the 1960s, the index only covers deaths since about 1962. It may include a few earlier records, but there hasn't been any concerted effort on the part of the SSA to enter older information.
There are many reasons why a person might not be in the SSDI:
If you do find someone, the data can provide clues for further research.
Don't rely just on the SSDI. As a genealogist you want to check the original records whenever possible. Order a copy of the original Social Security number application from the SSA. It will have the applicant's name, complete birth date and place, and often the parents' names. Since several states did not require birth certificates until the early or mid-1900s, the SSN application data could be used in lieu of a birth certificate.
The SSDI can point you in the right direction for gathering additional information, including ordering a copy of a death certificate from a state or county vital records office. The information on each state's death certificate varies, but unless the actual record is ordered you won't know.
How did I know this? Not from memory. I simply clicked on HELP in the Social Security Death Index viewer supplied with Ultimate Family Tree Platinum. It not only has tips on how the search functions, but also has information on the content of the data. Sometimes HELP really is help!