When it comes to genealogy, primary sources should always be sought. Unfortunately, it is not always possible to find primary sources. In this case it is important to try to find more than one secondary source to confirm that the information is correct.
Will or inventory
Basically, information that was created at the time that the event occurred is usually most accurate. Chances are that the person giving the information knew the facts. However, there are exceptions. On early census records, who gave the information is not noted. It may have been a neighbor and they may not have known the correct information. Death certificates often contain incorrect information as well, always look at the certificate to see who was providing the facts.
Secondary sources provide information at a date later than the time that it occurred. There are many documents that are not primary sources.
Death certificate- while a death certificate is a primary source for the date of death, other information on the certificate such as date of birth, place of birth and parents may or may not be correct and are therefore secondary sources.
Census- The people living in the house on that given date is a primary source other information is not necessarily correct unless the census shows the name of the person providing the information.
Burial record: The only information that is for sure is the date the burial took place and even that depends on the accuracy of the person who provided the information.
Family bibles- If it is a bible that dates to the time of the first entry then it is a primary source but if the information was filled in at a later date, it is secondary.
Family tradition/oral history- This is a great place to start but are information is suspect unless backed up with a primary source.
What to do if there are no primary sources
If primary sources are unavailable, secondary sources become the only option. It is important to try to find more than one secondary source and if they are conflicting it just complicates the search. Be creative when considering where secondary sources may be found, it may be in town records, personal letters or diaries and even newspapers. Obituaries can be very revealing.
Two final notes; always write down or copy any information found for individuals with the correct surnames even if the first name is not familiar. Later research may provide the relationship. Most importantly, always site the source, include location, page, exact information and who provided the information if it is verbal.